The Book of the Righteous
It totally changed the way I look at Clerics, Paladins, and gods and religion in RPGs forever. Even as a 20+ year RPGer, it never really clicked for me until I read that book. I always felt that the way RPGs approached clerics and religion in general was as something that got in the way of the game, instead of as an integral part of the world, but I just never saw anything that got my wheels turning to change that. This was reinforced in earlier editions of D&D by having all clerics cast almost the same exact spells no matter what, save if you were "Good" or "Evil". You didn't even need a god, just the fact that you were someone honoring "the Gods", faceless beings that they were. Even Deities & Demigods gave you little more than stats, and that was only so you could fight them.
At least in 3e you have a mechanic that allows for a slight diversity in spellcasting, though still extremely lacking. If you're lucky, you get two spells per level different than the cleric across the street.
That book doesn't really address that problem, but it does create a meaningful interaction of deities and the orders and organizations that result from their dogmas. Even if you don't use them out of the book, it gives you a fantastic base to build from.
For the money, that book is huge, well written and edited, and worth every penny. Easily Green Ronin's best book and my favorite d20 book out there today.
Disclaimer: I in no way work for Green Ronin, know the writers, etc. I am just an old gamer who got just as excited about a book as when I got my 1st edition DMG when I was 11.