To give you an idea of my tastes, I'm mostly a Dungeon Master, rather than a player. Long lists of spells, prestige classes, feats etc. bore me to tears. And I'm not really all that interested in simple lists of monsters or new PC races, either, unless they are really brilliant - I'm more interested in presenting existing races and monsters in new and interesting ways. Instead, I'm interested in tools to help me build new and interesting worlds, locations, and adventures - toolkits that make me more flexible and give me more ideas. I like books that get me thinking on new aspects of settings that I hadn't thought of before.
So, which books would you suggest?