Okay, here is my crack at this. I have several ideas about this. One such way to go about this is that the book is written in some sort of language or tongue that regular mortals can not understand. Maybe it's the language of the Universe, or the lost 'Original Tongue' and only the 'Guardians of Truth', an ancient race of whatevers sworn to protect it, can understand this strange language and alter the book. So your BBEG has managed to either persuade/corrupt/posses/coerce one of the Guardians into adding to the book and *BAM* demi-godhood.
Another route you might want to take is the metaphorical route. Maybe the Book of Truth is not a literal "Book" but perhaps something else entirely, but still serves the same function as the book as being a fulcrum of reality. I've always enjoyed stories that do this, though I know it is very hard to do, but if you can it has great results. An example is the Uncharted series of video games. In the first game, El Dorado is not a golden city, but rather a giant golden sarcophagus, cursed to turn whoever opens it into a rabid, deranged worshiper of it. So in essence, whoever tries to steal El Dorado winds up becoming its protector. In the second, the fabled Cintamani Stone turns out to be the resin from the Tree of Life. Granting the consumer of it extended life and vitality from its rich nutrients.
Overall though, I think you've got a decent idea for an adventure. Could you perhaps tell us a bit more about your BBEG? It might help us think of how you could flesh out this idea a bit more.