I don't see why this couldn't be possible. Whenever you open up a book in game, you get a small window. On that window is a title bar (for the name of the book) and back/forward buttons (if there are multiple pages.) I assume that the text for all WoW books is in a huge file somewhere that gets accessed by the game when you click on a book.
A book addon should be able to use similar coding, only it's executed when you hit a button, instead of a book graphic in the game. Then it can point to whatever file you want as the book. Perhaps use an easier to read font >.>.
One word of warning, DO NOT USE COPYRIGHTED BOOKS if you are going to release this addon. If you (or someone else) actually makes this addon, there are thousands of books that are now in the public domain that you can use.
http://www.gutenberg.org/wiki/Main_Page
Has all of the books you could ever want.
I really like this idea, and would be happy to contribute with adding books, provided an easy to understand LUA template was made.