Minecraft wiki entry for Development_versions, associated text: Editable Books – Book and quill, crafted using a book, feather, and ink sac
When a book and quill is made, it can be right-clicked to open a screen that allows you to type the book.
Pressing "Done" will save whatever was typed and allow you to edit the book again later, yet will not "sign" it.
When "signing" a book, it will allow you to choose a title up to 16 characters long and disable editing of the book, the Book and quill then become a written book, with the icon changing along with the label.