Get the illustrated version.
I read a copy that was loaned to me. The writing is simple but the pace is very fast. One thing that bothered me is that the main characters are running on very little sleep and very little food, so how can they possibly be thinking clearly and not be at each others' throats during the more stressfull episodes?

Its essentially a murder mystery with several small puzzles along the way, kinda like the PC games that were popular in the 90's like "7th Guest" and "Myst". The reader is presented with a puzzle, the characers solve it, and they move on to the next clue/puzzle. Its entertaining to try and solve the puzzles before the main characters do.
And its a neat trip through the history of art and religion.