As in a recent thread it was mentioned as useful for Ars Magica, I thought I'd let folks know that there is a sale of Expeditious Retreat Press's A Magical Medieval Society: Ecology and Culture, which is available for just 3$ and with some D&D extras as a bonusl; see here. The Silk Road product may also be of interest to Ars Magica. The principal relevant product is Western Europe, which is not on the sale (it's available online, but is expensive as it's not on the sale - at 15$ for the PDF).
Silk Road is fairly useful for sagas considering a trip to the Silk Road (In search of Bonisagus?) or South of the Sun-- depending on how you want the adventure to roll. The first two chapters are fairly mechanics light and universally usable, three is thick with d20 specifics.
Chapters 4 & 5 are about money and trade-- subjects that might interest a merchant companion, but probably don't have a lot of meaning for magi-focused stories aside from helping show what is scarce and what is valuable in an area. (They did skip right over slaves and their possible prices-- something that I've only seen Conan the RPG and Exalted's Manacle and Coin discuss in any detail.)
Chapter 6 details the historical Silk Road and pretty much leaves off at the 11C through the 14C, as the Pax Mongolia...and more or less takes the fun out of it. But, given that they feel the strength of the Silk Road is the intermingling of cultures and that period was (in their brief opinion) pretty homogenous, they pretty much end the very scanty historic overview there. It does give some decent high-level detail on the cultures, routes and countries of the Road (The names on the maps are nice to have, I think) which are worthwhile, and the Appendix on some materials would be good for covenants looking to magically improve a mundane process for a wealth generation mechanism. The bibliography is good sized and provides a nice launching point for further research.
Overall? It's better than a pass through wikipedia on some points, but if you're not running d20 Ars (You're doing what?) then a lot of the material isn't worth it. The PDF is probably more appropriate, or really a troll through google and the library for Silk Road books.might be just as useful and probably more detailed in a lot of places, though far more cumbersome.