Well, for my side project, https://www.bilinual.com I needed version controlling for changes that are made on a book (fixed typos, ...) and I found GITenberg project. There are several interesting tools developed during the project accessible here: https://github.com/gitenberg-dev

It's not so much about just adding git to books, but git to the transcriptions of books from printed materials. There are errors in the transcriptions, many books are lacking formatting, and many books pre-date Unicode and are in ascii. Tracking the book's source files in git makes it easier to collaborate on making these changes, or seeing what changes have been made.