(Hopefully I'm not rehashing any old discussions I may have missed)

Since there are so many D20 products out there, I was curious if anybody had found any that were particularly useful/applicable to the Birthright campaign setting. I've kept my eye out for these, but they're not always easy to find. Personally I have found two very helpful books:

Dynasties and Demagogues, published by Atlas Games

This one is an obvious choice, as the subtitle (The Sourcebook of Political Intrigue) indicates. It pretty much has all the basics you would expect in a D20 sourcebook: feats, prestige classes, spells, and magical items that are all geared towards intrigue and political activities, but stops short of giving rulership rules which makes it mesh nicely with BR. It also provides some nifty rules for debates that D20 mirror combat.

A Magical Medieval Society: Western Europe, by Expeditious Retreat Press

This is a very unique book in the sense that it offers no feats, spells, classes, or magical items (There is a template for a "Magical Medieval King" that borrows from the same "Divine Right of Kings" mythology as Birthright does). What this book contains is everything you need to know about the middle ages and didn't know you needed to know. It is extremely detail oriented while birthright is very abstracted, so the complement each other well even if they don't line up 100% (see my discussion here for an example). However, it does cover pretty much everything that might expect to go on inside a law, guild, or temple holding.