This is another diverting instance of Eco playing around with history, mythology, and ideas like a kid with Legos or something. This one is nominally set in 12th Century Byzantium and Northern Italy, but pulls in all kinds of crazy stuff from the early Sorbonne to the court of Frederick Barbarossa to the Assasins to the greater part of a mythological bestiary. The main thrust is the use of the Prester John myth as a political tool for Barbarossa's attempts to expand his power, but as with most of Eco's work, people soon start believing their own tales, and the protagonists set off on a quest for John's mythical kingdom, where all kinds of mythological and quasi-historical madness ensues. If you like Eco's other stuff, you'll definitely enjoy this one too.