I know that the text within books are stored within the Localization files. From the looks of it, book objects have an OnUseAction in their Template that points to the localization file text to use.
Edit:
If you're asking how to let the player write in a book itself, then I have no idea. It's probably not possible since that would require changing the localization file dynamically within the game itself.
Last edited by Rhidian; 01/03/15 04:32 PM.