Rule of Four follows up on the success of The DaVinci Code in the historical, academic mystery genre. Where Rule of Four differs is that the puzzle takes a back seat to character development of the four (double entendre?) Princeton senior roommates and an elder generation of Renaissance scholars all pursuing the secrets of the 15th century Hypnerotomachia Poliphili (a real book). Also, while the main storyline essentially comprises a three day period (similar to The DaVinci Code), the pace is substantially slowed by the flashbacks that provide the necessary backstory. Rule of Four was enjoyable enough to finish on a cross-country flight, but Dan Brown is still the master of the genre.