I would have given this post a miss, Phil, except it's you; and I love the chance to influence a publisher whenever and however possible.
In answer your first question, I will add another vote for The Road of Kings. It immediately came to mind. That book was written by a true fan and will likely be the only book I ever buy in the Conan line. I hope to use it to convert an old 1985 Conan RPG module to a d20 D&D game without any of the horrid crunchiness of the Conan d20 OGL game. TRoK is more like a gazetteer, which is much more useful to me than a book of weird variant fantasy d20 rules.
Without going into as much detail, here are some examples of great fluffy books from my shelves (ones that I intend to keep): Mesopotamia, Mysteries of the Drow, Judge Dredd (all books), Gamemastering Secrets, Robin's Laws of Good Game Mastering, DragonStar Starfarer's Handbook, The Giant's Skull, Burning Shaolin, D&D Gazeteer, Alternity Gamma World, and about 70 issues of Dungeon--especially #94 with Omega World on the Polyhedron side.
Honorable mentions go to DragonStar Raw Recruits, Redline, Spellslinger, Grimm, Greyhawk the Adventure Begins, and Rifts.
The details are generally found in answer to your second question. A good fluff book for me is one that allows me to use my favorite game, d20 D&D, as easily as possible while enjoying the sword-and-sorcery core game or a neat alternative in another genre. It's a very difficult balance to achieve.
The single best example I can give you is Omega World (OW). That mini-game packs into about 40 pages what many games struggle to do or cannot do in several hundred pages. OW is full of new crunch, but it's concise and easy to use; and it's very portable. Also, the crunchy parts are chock full of fluffy bits that help explain and define the setting. Some of the one-liners are priceless. The (very) short descriptions of the Cryptic Alliances are gems. Most importantly, it has a toolkit for the DM to allow easy incorporation of NPCs & foes. It perfectly captures the feel of the classic Gamma Wordl game for me, and I think it's the best d20 game ever. I may even prefer it to D&D.
The runner-up for me is Judge Dredd. That game has a lot of fluff to draw upon: almost 30 years of comics; novels; video games; and a movie. I knew very little of it before I bought the game. I watched the movie and read about a dozen borrowed comics. I was hooked after reading the core rulebook. It has a good fluffy overview of the game world. The game then presents enough changes to d20 D&D to make it playable as a post-apocalypitc/science-fiction urban game but not so many so as to make it unplayably different. I can't describe it all, but I will give you an example of Defense Value. Instead of a complicated chart or class-based bonuses to calculate DV, JDd20 jus makes it 10 + total Reflex save for a simple & playable alternative to AC (and it's the only time I've seen players select Lightning Relfelxes for their characters!).
But as good as I found the game, the core book would have been the end for me had the publisher not printed several adventures shortly thereafter. With modules to run, I was motivated to pick up all the fluffy (& somewhat crunchy) sourcebooks, too. Now, I've got everything (except time and players, unfortunately). The publisher nicely avoided the all-too-common trap of just putting out a source or campaign book of new rules without adventures to play.
To sell me a book in this market, the idea has to be inspired (brilliant or off the hook, so to say), AND it has to be immediately and easily playable--usually with or as a pre-written adventure(s). Anything else I'll outright pass or just buy, read and resell. I simply don't have the time or the inclination, especially when I have enough material that gives me what I want to play indefinitely.