This was my first thought too. IMHO, it was really the first book from the d20 3.X era that got religion right in terms of making a living pantheon and series of cults. Fundamentally it got something incredibly basic right, namely, "you do realize that people will actually want to worship/venerate these gods, right?"Without question, Green Ronin's "The Book of the Righteous".
I forgot to list GURPS 3e Martial Arts. I am pretty sure that I got rid of it when I stopped playing GURPS, because it is not on my shelf with the other GURPS books on my shelf following a move (I have a small sliver of hope it is in a storage container in the shed with some non-rpg stuff, but I doubt it).I forgot Ultimate Martial Artist! As well as it’s equivalent for GURPS 2Ed!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.