Settings are difficult since each one is fluffy by definition. Personally I would suggest Oathbound, which is in print and pdf, because it is one of the most flexable settings. Even though it is set up to be a megagrind, there is plenty within the setting for everything else. Wildwood, the last in the line (though Greg might release something in the future), brings down the power level even though it is the deadliest domain of those in print.
The Second World Sourcebook, in print and pdf, covers building worlds with both d20 and d20 Modern. The setting within the book is okay, but the rules on organization and the pact system (free on his site) are worth their weight in gold.
As for fluffy supplements and ignoring monster and template books:
En Publishing's The Fantastic Science. Technomagic using rules similar to psionics and tons of examples. Has rules for d20 Modern and a different base class if you don't want to use the point system.
Atlas' Crime and Punishment and Dynasties and Demagogues.
If you are interested in the setting, AEG's Stargate line. The quality is truely amazing and is jam packed with ideas. It makes me think of a coherent Aria, if that means anything to you.
Tabletop, a pdf publisher, has fluff only environmental supplements. If you want to expand upon your descriptions of the forest, swamp, plains, urban or underground areas, these are for you.
As for magic, Misfit Studios, Plain Brown Wrapper, Plot Device and Distant Horizons, all pdfs in this case, have potentially useful varients that are fluffy. MS added material to the existing schools of magic. PD reorganized all the SRD spells into a color scheme and created new base classes that are very unlike most casters. PBW has 3 pdfs of spells that are all over the place in theme (even if the pdfs are entitled Blasphemous). DH has path magic. If you like the eldritch weaver from Green Ronin's Advanced Player's Manual, you will like DH's Paths of Power I & II.
Genjitsu has released 2 pdfs that might interest you. Metablades mixes magic and swordplay. Shaping the Self is my favorite pdf, period. It covers paragon and similar classes- those that change the nature of the character.
I will let others speak of most of Troll Lord's Gary Gygax series, but the one I suggest is Nation Builder. It is not written for d20, but the appendix is (ie most of it is fluff).
I can go on, but I want to see your feedback on what I have posted so far to give me an idea of what else to suggest.