The sheer Doorstopper size and through historical and scientific research that goes into every game he makes. The Civilization ones are peppered with historical analysis of the Tech Tree and anacdotes from history explaining the game mechanics. The one for Sid Meiers Pirates has information about the rotes for the Spanish Silver Train and Treasure Fleets that are half there for Copy Protection, but are historically accurate, as well as analysis of fencing postures, statistics on period-accurate ships, and writeups on various Caribbean towns of the era. There are also more footnotes then a college textbook in the back. This is a guy who takes Shown His WorkUp to Eleven!