Board Thread:Episode Discussions/@comment-1181222-20170809100834/@comment-734582-20170812110337

Crazylazyguy wrote: I mean he had plenty of time when he was on the go. There are 2 possibilities - Sasuke or Orochimaru. Nobody else was there when this was explained. Or Tobirama. Really, it's not that strange that a bunch of research like that could exist in the Hokage's private library. It also wouldn't be strange if there were public demands for declassification, since the MS was a big part of what led to the 4th war.

Sure it looks like a new book, but that doesn't mean it's all new info. Someone probably compiled declassified info into a new book.