The content inside of the books is more important than the book itself. The book may very well have some powers itself, but I think that the knowledge the book possesses is of deep importance. It isn't a spoiler because it's something we already know of, like many people guessed and used as the basis for theorizing that it was a Keyblade, it's a spoiler because it most likely has something to do with the in-depth plot of the next game.
The information in that book could detail the history of the Keyblade, of Light and Darkness, of the Nobody and The Organization, of the beginning of time, of the Knights in the secret ending, maybe give someone any knowledge they want to possess, etc. along with enhancing Zexion's abilities and the ability to copy/mimic/forge other's weapons
People have had books for weapons before, I mean Luxord used a deck of cards, who cares? From the scans we can see that Zexion can trap others within the pages of the book, use pages of it to slice/cut and create whirlwinds, etc.
