Since my Homebrew has no active deities, or clerics, and uses AE's spellcasting system, where there's no Arcane/Divine distinction, I initially passed on this book, however, I have found that in some cases certain PrCs work for NPCs in my campaign setting.
Black Flame Zealot- excellent Villain filling the Cultist, Assassin, and BBEG's right-hand-man roles.
Entropomancer- I've used one of these as a major BBEG in my own campaign. Thematically, very cool, but not terribly survivable, in my experience.
Evangelist- In a world without active/interfering gods, Evangelists (often Bards) will nonetheless preach, and gather a following.
Geomancer- My alterations of this class to make it fit with AE's spellcasting system are a bit too extensive to detail here, but suffice it to say, Spell Versatility allows multiclass spellcasters to make their casting Verbal-only by becoming more closely tied to Nature. I might consider letting a PC do this.
Pious Templar- For when you absolutely, positively, need to make it difficult for the PCs to get into a Temple.
Stormlord- More or less becomes a specialized order of Lightning-mages, rather than having divine affiliations. I might consider letting a PC do this.
Robert "Mostly Just NPCs Though" Ranting