Board Thread:Episode Discussions/@comment-1181222-20170809100834/@comment-734582-20170811185455

Crazylazyguy wrote: The book seems relatively new, I'd say that it was indeed Sasuke. After all, he was there when Tobirama explained everything about the Sharingan. It's probably declassified information, so it's probably Tobirama's (with some Orochimaru research thrown in).

I don't see when Sasuke had time to write that book.