I would really like to see some kind of bookshelf with the ability to pull a book, sending out a redstone signal! The book needed to pull could be customized, like you would be able to choose which one you could pull! In this way, you could make even cooler hidden entrances, and bookshelves would actually have a purpose!
I think that comparators should detect how much books are in the book shelf and send a redstone signal with a certain strength based on that (similiarly to cauldrons). You could still invert the signal to make it so that pulling a book out powers something.