Turjan
Explorer
I can imagine that they are hard to fine nowadays. With the exception of the Northern Crown books, John Nephew gave all his remaining Penumbra d20 books away for free to everyone who wanted to have some. That's one of the greatest ways to leave the d20 market that I have seenAnd I am officially annoyed! I canot find any of these books at my LGSs. Frustrating.
I'm going to have to hunt for them; I just know it.

I just looked which of the books I have are 3.5 versions: Crime & Punishment, Love & War, Sacred Grounds II, Seven Civilizations, Seven Serpents.
Dynasties & Demagogues is one of the best books in the line, but it's only useful if you want to run a 3.x game full of political intrigue.
The "En Route" series books are full of encounters that I mostly enjoyed. Seven Strongholds is a very nice location book. Seven Serpents delivers some ready to use dragons. Seven Cities are some drop-in locations from small to big, and I especially like the smaller ones, although you probably have already enough of those. Seven Civilizations is inspirational, but a bit hard to drop into an existing world. The Sacred Grounds books are somewhat hit and miss with their locations.
Occult Lore is a strange book. The Realm of Reason is outrightly stupid in a D&D context. I didn't like the first chapter about Alchemy, either, although the organization "The Golden Cabal" is quite nice. The Astrology chapter has some nice fluff, I liked the Pangunculus, but the classes are overcomplicated, too weak for all the loops you have to jump through. And so on. Much light and much shadow. Much crunch you will never use. Only get it if you get it cheap.