A Book is a crafting ingredient used to craft a bookshelf. It currently has no other use. In the PC/Mac version, you can use a book and quill to write a message in a book, like a sign, but with infinite room. Books can also be found in the chests of stronghold libraries.
3 Pieces of paper are required to craft a book. 3 books and 6 wooden planks can then be used to craft the Bookcase. In a future update the Enchanting Table could be added making the book and bookcase have more use.