In between building dungeons, doing character work and planning for Is This Foul? I've been fiddling a bit with an all psionics 3.5 setting. Nothing terribly serious, mind; I have enough games that I'd rather run than 3.5 that it won't see play for a while, if at all. (I am considering converting it to Swords & Wizardry, but that's a whole 'nother ball game.) It's just a fun side-activity when I get bored with my regular games.
In the process, I dug out Sandstorm, one of my favorite 3rd edition supplements. I've never been able to use it as extensively as I'd like, but bits and pieces have found their way into my games over the years. Mostly, I like reading it, and getting ideas.
It's representative of the type of sourcebook that I love the best. I don't have much use for things like Complete Divine or Arcane Power. Player sourcebooks aren't helpful to me as a GM or interesting to me as a player. And while I enjoyed reading Eberron (and still wouldn't mind playing in a game someday) there's generally so much that I rip out of a setting in the process of getting it to conform to my whims that it makes more sense to just whip up a setting from scratch, or something close to it.
Sandstorm has a lot of the obligatory feat and prestige class type crunch endemic to books of that period, and it implies a fair amount of setting detail. But it's all very modular. There are a lot of things that logically fit if you used them all in one game, but no rules as to how they interrelate, no politics, geography, or star NPCs. What it does have is a lot of crunch for building desert-ready characters, some great monsters with intriguing world and plot hooks attached, and a lot of detailed information on desert environments and dungeons. It's a toolbox for building a campaign.
That's what I want out of a sourcebook. Game-focused, researched information. Spells, classes, and monsters that suggest villains and organizations, rather than handing them to me ready made. If that material is interconnected by a theme and sub-themes -- the environment of the desert, its races and their cultures, and the lost civilizations responsible for some of its monsters and traditions -- all the better.
These days, I'm more interested in systems that don't suggest or require as many sourcebooks as does 3rd. But when I go back to it, those are the books I get the most use out of. The environment books, Deities and Demigods, the alternate power sources, The Book of Vile Darkness. And I find that the best parts -- the desert terrain information in Sandstorm, or the classification of different kinds of pantheons in Deities and Demigods, are fundamentally system neutral. I can use them for Swords & Wizardry or 4th edition or even Traveller or Vampire just as easily as I can for 3rd edition.