The book you're thinking about is Interstellar Players 2: Jihad Conspiracies, which includes a 9 page discussion of the Belt, though in classic Interstellar Players unreliable/conspiracy fashion.
Terra (and the Belt) are vaguely touched on in a few other books, in particular JHS: Terra, but they're fairly ignored, for the most part. Terra was always sort of "outside" the standard BattleTech universe; and especially the Belt, which (as much as we can see it) feels like a different scifi subgenre altogether (especially since settling asteroid belts seems to have become passe with the invention of the K-F drive).