I second the good old AD&D Core books, just because you can oftentimes just feel Gary Gygax's excitement, back at the forerunner of the system...
Plus, just reading through the oftentimes cruel magic items is a treat in and of itself.
As for flavor, Dwarf Fortress is nice, though that is a computer game.
However, for shear world building, and something to take notes from, may I recommend the Malazan Book of the Fallen? Looooong series of doorstoppers, where the line between mortals and gods is just plain frayed. I mean, when there is a character whose nickname is The Emperor of a Thousand Deaths, and one character comments that they're exaggerating, it's only been a hundred or so...
Also, it's just plain great to grab ideas for religion from.
Also, Redwall, for the nostalgia and fun it brings me. I've always thought it should be adapted into an RPG; the closest thing flavorwise would have to be Mouseguard, though.
Also, I like looking at Risus, just for the clarifying simplicity and the reminder that rules aren't important next to having an awesome and fun story/game. Perspective, really.
But I have deep, abiding love for our good old friend Tome of Magic, especially the Binder section, which has absolutely awesome flavor, and just ends up generating a ton of ideas.
Also, Die unendliche Geschichte, for our German readers
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