If you don't know Hindi and Sanskrit, the learning of Hindi at first would the best and most reasonable option.
Hindi is very easy and you can learn Hindi in 6 months (studying about 5-6 hours per day), Sanskrit is au contraire, too complicated and even with acute diligence it will take about 3 years to learn.
The second advantage with Hindi is that if you read Vaishnava-Books in Hindi, you will also learn Sanskrit words, because usually all the translators from Sanskrit to Hindi leave Sanskrit words intact.
It also means, that you should use Sanskrit Vocabulary, when you read for example "Shree Vishnu-Purana" or "Shree Ujjvala-Nilamani" in Hindi.
Not all the books are included in this part of my blog, therefore don't forget that Shree Shreemad Bhaktivedanta Narayana Maharaj and Shree Vallabha-Sampradaya have really outstanding E-BOOK libraries (Shree Shreemad Bhaktivedanta Narayana Maharaj E-BOOKS in Hindi and Sanskrit and Shree Vallabha-Sampradaya E-BOOKS repository in Sanskrit, Hindi and Gujarati).
I will add some Sanskrit Grammars to this part of my blog later.
1. SRI VISHNU SAHASRANAMA WITH THE BHASHYA OF SRI PARASARA BHATTAR" [Sanskrit-English] #translated by prof. A. Srinivasa Raghavan# ~1983~ http://vishnudut1926.blogspot.ru/2017/12/sri-vishnu-sahasranama-with-bhashya-of.html
2. VISHNUTATVAVINIRNAYAH [VISHNU-TATVA-NIRNAYA] by ~OM VishnuPad Shree MadhvAcharya~ #translated by professor K.T. Panduranagi# [Dvaita Vedanta] 1991 - http://vishnudut1926.blogspot.ru/2017/12/vishnutatvavinirnayah-vishnu-tatva.html
3. "SHREE LAKSHMI-TANTRA", SANSKRIT, PANCHARATRA AGAMA, Adyar Library, reprint 1975 - http://vishnudut1926.blogspot.ru/2013/10/shree-lakshmi-tantra-sanskrit.html
4. "SHREE LAKSHMI SAHASRAM", SANSKRIT WITH ENGLISH EXORDIA AND YANTRAS (at the last pages of the book), Shree Ramanuja Seva Trust, 1988 - http://vishnudut1926.blogspot.ru/2013/08/shree-lakshmi-sahasram-sanskrit-with.html
5. "SRI SUBODHINI" - Om VishnuPad Vallabhacharya Tika on "Shreemad Bhagavatam", SANSKRIT-HINDI, 39 files (2,8 GB) - http://vishnudut1926.blogspot.ru/2015/03/sri-subodhini-om-vishnupad.html
2. VISHNUTATVAVINIRNAYAH [VISHNU-TATVA-NIRNAYA] by ~OM VishnuPad Shree MadhvAcharya~ #translated by professor K.T. Panduranagi# [Dvaita Vedanta] 1991 - http://vishnudut1926.blogspot.ru/2017/12/vishnutatvavinirnayah-vishnu-tatva.html
3. "SHREE LAKSHMI-TANTRA", SANSKRIT, PANCHARATRA AGAMA, Adyar Library, reprint 1975 - http://vishnudut1926.blogspot.ru/2013/10/shree-lakshmi-tantra-sanskrit.html
4. "SHREE LAKSHMI SAHASRAM", SANSKRIT WITH ENGLISH EXORDIA AND YANTRAS (at the last pages of the book), Shree Ramanuja Seva Trust, 1988 - http://vishnudut1926.blogspot.ru/2013/08/shree-lakshmi-sahasram-sanskrit-with.html
5. "SRI SUBODHINI" - Om VishnuPad Vallabhacharya Tika on "Shreemad Bhagavatam", SANSKRIT-HINDI, 39 files (2,8 GB) - http://vishnudut1926.blogspot.ru/2015/03/sri-subodhini-om-vishnupad.html
6. "SHREE VISHNU-PURANA", SANSKRIT-HINDI, Mula Shloka aur Hindi-Anuvadsahit, Munilala Gupta, Motilal Jalana Gitapres, Gorakhapur 625 pages, NEW SCAN - http://vishnudut1926.blogspot.ru/2013/05/shree-vishnu-purana-sanskrit-hindi-mula.html
7. "ISHAVASYOPANISHAD BHASHYA" by SHRI VEDANTA DESIKA, sanskrit-eng by Dr K C Varadachari, 1975 - http://vishnudut1926.blogspot.ru/2015/11/ishavasyopanishad-bhashya-by-shri.html
8. "SHREE VISHNU-PURANA", SANSKRIT, with_Vishnuchittiyam, PB Annangaracharya, 1972 - http://vishnudut1926.blogspot.ru/2013/05/shree-vishnu-purana-sanskrit.html
9. "SANAT-KUMARA-SAMHITA OF THE PANCARATRAGAMA", SANSKRIT, edited by Pandit V Krishnamacharya, Adyar Library, 1969 - http://vishnudut1926.blogspot.ru/2013/05/sanat-kumara-samhita-of-pancaratragama.html
10. "THE VAISHNAVA UPANISHADS", SANSKRIT, Adyar Library, 1923 - http://vishnudut1926.blogspot.ru/2013/08/the-vaishnava-upanishads-sanskrit-adyar.html
11. "THE BRAHMASUTRA BHASHYA OF MADHVAACHARYA" with glosses of Shree Shreemad JayaTirtha, Shree Shreemad VyasaTirtha and Shree Shreemad RaghavendrAcharya (1911, 1915, 1920) - http://vishnudut1926.blogspot.ru/2013/08/the-brahmasutra-bhashya-of.html
12. "KALKI-PURANA", HINDI-SANSKRIT - http://vishnudut1926.blogspot.ru/2013/06/kalki-purana-hindi-sanskrit.html
13. "BHAGAVAD-GITA (VYAKHYAMRTASARITA)", HINDI, Ramanuja Sampradaya - http://vishnudut1926.blogspot.ru/2013/06/bhagavad-gita-vyakhyamrtasarita-hindi.html
14. "THE BHAGAVAD GITA BHASHYA OF SHREE MADHVACARYA", ENGLISH-SANSKRIT, translated B.N.K. Sharma, Anandatirtha Pratisthana, 1989 - http://vishnudut1926.blogspot.ru/2013/06/the-bhagavad-gita-bhashya-of-shree.html
15. "VISHNUSAHASRANAMA BHASHYA" by Shree Satya Sandha Yatiraja, SANSKRIT Madhva Sampradaya, K.G. Kalkoti, Nagaraja Rao, Tiruchirapalli, 1972 - http://vishnudut1926.blogspot.ru/2013/06/vishnusahasranama-bhashya-by-shree.html
16. "VASUDEV MAHATMYAM" ENGLISH-SANSKRIT published by Mahant Purani Swami Dharmnandan Dasji, Shree SwamiNarayan Temple Bhuj - http://vishnudut1926.blogspot.ru/2013/06/vasudev-mahatmyam-english-sanskrit.html
17. "PANCARATRA RAKSHA" by Shree Shreemad Vedanta Desika, SANSKRIT, Adyar Library, 1942 - http://vishnudut1926.blogspot.ru/2013/06/pancaratra-raksha-by-shree-shreemad.html
18. "KRISHNA MAHATMYAM" FROM SKANDA PURANA, SANSKRIT-TELUGU, Tirumala Tirupati, Devasthanams, 1997 - http://vishnudut1926.blogspot.ru/2013/06/krishna-mahatmyam-from-skanda-purana.html
19. "BRIHAT BRAHMASAMHITA", SANSKRIT, 1912, Anandashram Samskrita Granthavali - http://vishnudut1926.blogspot.ru/2013/06/brihat-brahmasamhita-sanskrit-1912.html
20. "SHREEMAD BHAGAVATAM" ("BHAGAVATA PURANA"), SKANDHAS 1-12, HINDI-SANSKRIT, Enhanced Scan - http://vishnudut1926.blogspot.ru/2013/07/shreemad-bhagavatam-bhagavata-purana.html
21. "BHAGAVAT MAHAPURANA", Gita-Press, Gorakhpur, SANSKRIT-HINDI - http://vishnudut1926.blogspot.ru/2015/03/bhagavat-mahapurana-gita-press.html
22. "SHREE VENKATACHALA ITIHASA MALA" ("SHREE VENKATESHA ITIHASAMALA"), SANSKRIT WITH ENGLISH EXORDIA-SYNOPSIS, Tirumalai Tirupati Devastanam Series, 1937 - http://vishnudut1926.blogspot.ru/2013/07/shree-venkatachala-itihasa-mala-shree.html
23. "SHREE BRAHMA-VAIVARTA-PURANA" (HINDI) with Stotras and Kavachas in Sanskrit (starting from page 737) - http://vishnudut1926.blogspot.ru/2013/09/shree-brahma-vaivarta-purana-hindi-with.html
24. "Shree Krishna-Karnamrita" by Shree Shreemad BilvaMangala Thakur (Leela Shuka) SANSKRIT WITH ENGLISH EXORDIA, 1900 edition ("Shree Vani Vilas Sanskrit Series", NO. 19) - http://vishnudut1926.blogspot.ru/2013/10/shree-krishna-karnamrita-by-shree.html
25. "SHREE VENKATACHALA MAHATMYAM", 2 Volumes, UNIQUE EDITION, Sanskrit, Tirumala Tirupati - http://vishnudut1926.blogspot.ru/2014/01/shree-venkatachala-mahatmyam-2-volumes.html
26. "VISHNU-KHANDA" from Skanda-Purana including "SHREE VENKATACHALA MAHATMYAM", SANSKRIT - http://vishnudut1926.blogspot.ru/2013/10/vishnu-khanda-from-skanda-purana.html
27. 34 VAISHNAVA-SAMHITAS IN SANSKRIT AND TELUGU (with "Shree Parama-Samhita" and "Agama-Pramanyam" by Shree Shreemad Yamuna-Acharya in ENGLISH) - http://vishnudut1926.blogspot.ru/2013/11/34-vaishnava-samhitas-in-sanskrit-and.html
28. PARAMASAMHITA OF THE PANCHARATRA [Sanskrit-Eng] #by RajaSevasakta Dewan Bahadur Dr S KrishnaSwami Aiyangar# ~Baroda Oriental Institute~ [1940] - http://vishnudut1926.blogspot.ru/2016/02/paramasamhita-of-pancharatra-sanskrit.html
29. "HAYASHIRSHA PANCARATRA ADI KHANDA", Sanskrit with English Exordia and Addendum 1976 - http://vishnudut1926.blogspot.ru/2014/01/hayashirsha-pancaratra-adi-khanda.html
30. "GOPALA SAHASRANAMA STOTRAM" with Bhagavadgunaratna Petika Tika, Sanskrit, Tirumala Tirupati, 1985 - http://vishnudut1926.blogspot.ru/2014/01/gopala-sahasranama-stotram-with.html
31. "SANKARSHANA SAMHITA Narada Pancaratram" SWAMINARAYANA AKSHARA PITH SANSKRIT 2004 - http://vishnudut1926.blogspot.ru/2014/01/sankarshana-samhita-narada-pancaratram.html
32. Gautamiya Tantra Sanskrit - http://vishnudut1926.blogspot.ru/2014/02/gautamiya-tantra-sanskrit.html
33. "SHREEMAD DEVI BHAGAVATAM" ("SHREE DEVI BHAGAVATA PURANA"), Sanskrit, 826 pages - http://vishnudut1926.blogspot.ru/2014/02/shreemad-devi-bhagavatam-shree-devi.html
34. "KRAMADIPIKA" by Kashmirika Keshava Bhatta with Vivarana Tika by Govinda Bhattacharya SANSKRIT-HINDI edition by Sudhakar Malaviya, Varanasi, 1989 - http://vishnudut1926.blogspot.ru/2016/01/kramadipika-by-kashmirika-keshava.html
35. "Krama Dipika" - Vaishnava Agama with Gopala Mantra Glosses and Clarifications, Sanskrit, Srirangar 1929 - http://vishnudut1926.blogspot.ru/2014/07/krama-dipika-vaishnava-tantra-with.html
36. "SHREE KRISHNA YAMALAM MAHATANTRAM" (श्रीकृष्णयामलं महातन्त्रम्) , Sanskrit, Varanasi Tantrika Text Series NO 06, Prachya Prakashan, Varanasi, 1992 (WITH HINDI PRASTAVANA) - http://vishnudut1926.blogspot.ru/2014/07/shree-krishna-yamalam-mahatantram.html
37. श्री समोहन तन्त्रं पार्वतीश्वरसंवादे श्रीगोपालसहस्रनामस्तोत्र भाषाटीकासहित + श्री गोपाल सहस्र नामावलिः (HINDI-SANSKRIT) - http://vishnudut1926.blogspot.ru/2015/01/hindi-sanskrit.html
38. "SHREE NAMRATNAKHYA STOTRA" #Shree RaghuNathaLala Rachita#, Sanskrit-Hindi - https://vishnudut1926.blogspot.ru/2016/12/shree-namratnakhya-stotra-shree.html
39. "VAISHNAVA PANCHARATRA AGAMA KATIPAYA PAKSHA" (HINDI-SANSKRIT), by Dr. Ram Pyare Mishra, 1st edition, 1994 - http://vishnudut1926.blogspot.ru/2017/04/vaishnava-pancharatra-agama-katipaya.html
40. "RADHA-RASA-SUDHA-NIDHIH" [Hindi-SaNskRit], edition by Shri Haridas Shastri, Kalidah-Vrindavan-Mathura, 2007 - http://vishnudut1926.blogspot.ru/2017/04/radha-rasa-sudha-nidhih-hindi-sanskrit.html
41. "SHREE VARAHA-PURANA" (Sanskrit, with Adhyayas numbers embedded into bookmarks for easy search) - http://vishnudut1926.blogspot.ru/2017/04/shree-varaha-purana-sanskrit-with.html
42. GADYATRAYA #ShriBhagavadRamanujaMunindra Virachita# ^SharanagatiGadya-ShriRangaGadya-ShriVaikunthaGadya^#Sanskrit-Hindi# - http://vishnudut1926.blogspot.ru/2017/06/gadyatraya-shribhagavadramanujamunindra.html
43. "Shri Vaishnava-Kritya-Darpana" [Sanskrit-Hindi] - http://vishnudut1926.blogspot.ru/2017/07/shri-vaishnava-kritya-darpana-sanskrit.html
44. "SHRI RADHA-UPASUDHA-NIDHI" #MahaPrabhu ShriHita KrishnaChandra# Hindi Bhavanuvad Sahita - http://vishnudut1926.blogspot.ru/2017/07/Radha-UpaSudha-Nidhi-Hindi.html
45. "SHREE MANTRARTHA MANJARI" by OM VishnuPad Shree Raghavendra Tirtha #Rayaru, Shree Madhva-Sampradaya# [Sanskrit] - http://vishnudut1926.blogspot.ru/2017/12/shree-mantrartha-manjari-by-om.html
46. "AGASTYA-SAMHITA" [Lord RamaChandra Agama] from Shree Vaishnava-PanchaRatra (one of the Sita-Rama Tantras) #Hindi-Sanskrit# - http://vishnudut1926.blogspot.ru/2017/12/agastya-samhita-lord-ramachandra-agama.html
47. "Dashashlokee by Shree NimbarkAcharya" #SANSKRIT# with a commentary called LaghuManjusha [by Shree Giridhar Prapanna] - http://vishnudut1926.blogspot.ru/2017/12/dashashlokee-by-shree-nimbarkacharya.html
48. "RAGHUNATHA VIJAYA" of Sri Raghuvaryatirtha [Sanskrit] #University of Mysore# 1985 - http://vishnudut1926.blogspot.ru/2017/12/raghunatha-vijaya-of-sri.html
49. "YATHARTHAVYATIBHUSHANAM" #Ramanuja TatAcharya# [SANSKRIT] ~SriRangam Srimad Andavan Ashramam~ [1994] - http://vishnudut1926.blogspot.ru/2017/12/yatharthavyatibhushanam-ramanuja.html
50. "NYAYAVIVARANAM OF SRI ANANDATIRTHA" [Sanskrit] #Raghavendra Enterprizes# 2001 - http://vishnudut1926.blogspot.ru/2017/12/nyayavivaranam-of-sri-anandatirtha.html
51. "Shree SaptaGitam" - vintage edition by VenkatEshwara Press... - http://vishnudut1926.blogspot.ru/2017/12/shree-saptagitam-vintage-edition-by.html
52. "DAYASAHASRAM" by Sri Nigamantha Mahadesika [SANSKRIT] edited by BalaDhanvi Jaggu Sudarshanachar #1975# - http://vishnudut1926.blogspot.ru/2017/12/dayasahasram-by-sri-nigamantha.html
53. "VAISHNAVA-SANDHYA" #Sanskshipta Tantrik Paddhati of Nimbarka-Sampradaya# [Hindi] - http://vishnudut1926.blogspot.ru/2017/12/vaishnava-sandhya-sanskshipta-tantrik.html
42. GADYATRAYA #ShriBhagavadRamanujaMunindra Virachita# ^SharanagatiGadya-ShriRangaGadya-ShriVaikunthaGadya^#Sanskrit-Hindi# - http://vishnudut1926.blogspot.ru/2017/06/gadyatraya-shribhagavadramanujamunindra.html
43. "Shri Vaishnava-Kritya-Darpana" [Sanskrit-Hindi] - http://vishnudut1926.blogspot.ru/2017/07/shri-vaishnava-kritya-darpana-sanskrit.html
44. "SHRI RADHA-UPASUDHA-NIDHI" #MahaPrabhu ShriHita KrishnaChandra# Hindi Bhavanuvad Sahita - http://vishnudut1926.blogspot.ru/2017/07/Radha-UpaSudha-Nidhi-Hindi.html
45. "SHREE MANTRARTHA MANJARI" by OM VishnuPad Shree Raghavendra Tirtha #Rayaru, Shree Madhva-Sampradaya# [Sanskrit] - http://vishnudut1926.blogspot.ru/2017/12/shree-mantrartha-manjari-by-om.html
46. "AGASTYA-SAMHITA" [Lord RamaChandra Agama] from Shree Vaishnava-PanchaRatra (one of the Sita-Rama Tantras) #Hindi-Sanskrit# - http://vishnudut1926.blogspot.ru/2017/12/agastya-samhita-lord-ramachandra-agama.html
47. "Dashashlokee by Shree NimbarkAcharya" #SANSKRIT# with a commentary called LaghuManjusha [by Shree Giridhar Prapanna] - http://vishnudut1926.blogspot.ru/2017/12/dashashlokee-by-shree-nimbarkacharya.html
48. "RAGHUNATHA VIJAYA" of Sri Raghuvaryatirtha [Sanskrit] #University of Mysore# 1985 - http://vishnudut1926.blogspot.ru/2017/12/raghunatha-vijaya-of-sri.html
49. "YATHARTHAVYATIBHUSHANAM" #Ramanuja TatAcharya# [SANSKRIT] ~SriRangam Srimad Andavan Ashramam~ [1994] - http://vishnudut1926.blogspot.ru/2017/12/yatharthavyatibhushanam-ramanuja.html
50. "NYAYAVIVARANAM OF SRI ANANDATIRTHA" [Sanskrit] #Raghavendra Enterprizes# 2001 - http://vishnudut1926.blogspot.ru/2017/12/nyayavivaranam-of-sri-anandatirtha.html
51. "Shree SaptaGitam" - vintage edition by VenkatEshwara Press... - http://vishnudut1926.blogspot.ru/2017/12/shree-saptagitam-vintage-edition-by.html
52. "DAYASAHASRAM" by Sri Nigamantha Mahadesika [SANSKRIT] edited by BalaDhanvi Jaggu Sudarshanachar #1975# - http://vishnudut1926.blogspot.ru/2017/12/dayasahasram-by-sri-nigamantha.html