The Rule of Four

I finally finished my book. It took me a lot of RER trips back and forth to work. It's a good book although I'm a little disappointed about how the Hypnerotomachia was puzzled out. It was a bit too easy for Paul and Tom to solve it compared to all the crap Tom's dad and his so called friends went through. But still, it was very entertaining.

I still have a few questions to ask:
What's with the different cover colors?
The second one is for Sabrina in the comments, this question concerns the end of the story, so if you still want to read that book, don't read the comments! Thanks ;)