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?"
Honorable Mentions: mainly for being settings that influenced how I think about settings, and I respect a certain degree of coherency of the the first two listed settings in particular.
* Eberron Campaign Setting Book
* Blue Rose (and the True20 book)
Also, Fate Core (and Accelerated). I think that Fate Accelerated presented a more concise depiction of how Fate works, so it made a lot of it digestible for me when I was first looking into the system. I don't think that the writing is all that great, as [MENTION=4937]Celebrim[/MENTION] mentions, but once you get past the "pedestrian" writing, there are a lot of gems. I do think that some of the better explanations of the basic rules actually come from some of their other games (e.g., Atomic Robo, Young Centurions) or from 3rd party published settings (e.g., Jadepunk).
I forgot Ultimate Martial Artist! As well as it’s equivalent for GURPS 2Ed!
Both books make HTH combat- and the characters who focus on it- so much less generic. And as I recall, one edition of UMA even had a conversion system to use it with D&D. Never got to use it, though it inspired me to try.
Galaxy Guide 9 for the Star Wars RPG by West End Games probably tops the list for me. It’s got popular music, cocktails, gadgets, and a ton of information on what life is like under the empire. The Galaxy Guides are all pretty damn good, but 9 is th best.
4e’s Player Options: Heroes of The Feywild. Explores the Feywild as a setting, including locations, characters, factions and powers, with fun “Bard’s Tale” sidebars that present legends and fairy tales related to the info being presented.
And, it has a chapter on building a backstory using skill tests and “choose your own adventure” style storytelling. I just used it to make my Half-elf Fey-lock for an upcoming CoS game, and it worked just as well for 5e as it did for 4e, barring the couple of instances where a skill doesn’t exist anymore. Generally pretty easy to figure out though. Thevery=thieves tools or slieght of hand, heal=medicine, etc.
Cubicle 7’s The One Ring is probably the most gorgeous RPG book I’ve ever seen, and the matching of mechanics to lore is perfect, without feeling restrictive. Especially the second edition with better organization and improved index.
4e’s Shadowfel: Gloomwrought and Beyond is a beautifully done adventure book thAt also serves as a setting book for the Shadowfel. If only the Heroes of Shadow book had matched it.
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).
Cults of Terror. I think Trollpak is better, but that's a boxed set with three books and other things so doesn't qualify as a single book. In no particular order after that since I can't restrain myself to just one, Gurps Traveller: Nobles, Shadows on the Borderlands, The Heart of the Wild, Pendragon 4e, River of Cradles, Fate Core, Star Trek Adventures, Darkening of Mirkwood, Tianxia
RC because...well, a pad of paper, some dice, and a pencil and you can run a game for DECADES and never need anything else.
Hackmaster 4th because it has pretty much all of what's in the 1e DMG, plus a bunch more, and is sprinkled with "attitude humor" that just makes it a fun read.
Honourable Mention: This goes to Powers & Perils (it's a boxed set, but could have been made into a single book). Man...P&P has given me so much fun and entertainment! ALMOST (as in short by about 2 years I think) as long as Basic D&D.