Brahmagupta's Brāhmasphutasiddhānta
Correctly Established Doctrine of Brahma
https://archive.org/details/628ceBrahmasphutasiddhantaBrahmaguptaParts1-4BySudhakaraDvivedin1902
I would like to thank the University of Chicago Library for making their microfilm of the book available so I could have it scanned in Chennai India, where it was converted into hundreds of single TIF images. I then cropped cleaned and edited these images to produce a downloadable PDF version.
After sharing this with Indian mathematics professors who read Sanskrit, and having been helped by them, I have really started to get to appreciate how Brahmagupta intended arithmetic to work!
This book can also be studied with a 1966 edition of Brahmagupta's Brāhmasphutasiddhānta with Hindi commentary at:
https://archive.org/details/Brahmasphutasiddhanta_Vol_1
https://archive.org/details/Brahmasphutasiddhanta_Vol_2
https://archive.org/details/Brahmasphutasiddhanta_Vol_3
https://archive.org/details/Brahmasphutasiddhanta_Vol_4