When debugging issues with a book it is nice to know exactly what sources the book was built from. It would be great if the hash of the git commit the book was built from was displayed somewhere.

One possibility is to display it on the Author Tools page. Another possibility would be to actually embed it somewhere in the book itself so when browsing the book you can check what version it was built from. (This would also help debug caching issues where some people are seeing an old version of the book because some server hasn't been updated or whatever.)

Ideally, we could have both.