The 2e Von Richen Guides were pretty revolutionary at the time because they treated monsters as characters in their own right, fully customizable and diverse. They'll probably seem less revolutionary now that that has become the norm, especially post 3e. However, you may find the guides to Vampires, Ghosts, and Created as inspirational. And though WotC disavows it, the guide to the Vistani is pretty good as well, and by no means was meant at the time disrespectfully and if anything the reverse. The Vistani were clued in, woke sorts that saw the big picture.
The 3e books by S&S draw heavily on the 2e texts and are in my opinion some of the best 3e supplements that were written. But you really only need the core rulebook, as the 2e texts are at least as good at imparting the flavor of the setting - rich settings being the real trademark of 2e.
