As a general quality, I loved how 3rd edition books by and large felt like tomes - large books with small text with lots of random, and often useless, info in them, but without the slick (and often boring) streamlined feeling of 4E books. I felt that 3E did a good job recaptures at least some of the elements that made 1E books fun to browse, but for which 2E was a bit less interesting.
As for specific books, in that regards the 3.5E Player's Handbook is terrific, and the Dungeon Master's Guide is pretty good. But for me, as a setting junky, my favorite 3.x book is the Forgotten Realms Campaign Setting hardcover which, along with the 1987 gray box, is the best FR product (imo) and is simply the most beautiful, complete setting product that WotC or TSR has every produced. A master work, really. Also, a lot of their later theme books were quite good (the various codexes and books that looked at weird creatures and alternate rules).
Beyond WotC, I have great appreciation for The Book of Righteous Might which has similar qualities to what I describe above: more ideas than should fit into its page count, which is a good thing and what I love about RPG books (think the 1e DMG as the archetype of this quality).