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The locked book is in the library in the bookshelves right next to where Henry is working, but you can't get to it until you get into the room at the end of the secret passage (the one that you needed the key to get into). There is a book of stories written by Bruno in there that gives clues to several of the things that you are going to need to do before you solve the mystery, and one of them is the clue to the combination of the book.
Once you have Bruno's book and you've read the stories (the clue for the book is called "The Librarian's Tale"), you can get to the book.