Originally Posted by
Tzardok
If you like the planes, most of the Planescape books from 2e are very good and expound on a lot that Manual of the Planes only describes superficially. Planescape books are also written in-character by people in the setting, making them entertaining to read, going deep into the lore and opening up the chance for contradictions and the narrators being wrong. I can especially recommend Planes of Chaos, Planes of Law, Planes of Conflict, The Inner Planes, On Hallowed Ground, Guide to the Astral and Guide to the Ethereal. A Player's Primer to the Outlands is also supposed to be very good, but I haven't had a chance to read it yet.
Amongst 3.5 books, I can also wholeheartedly recommend Fiendish Codex 1, which is the closest you'll get to a 3.5 Planescape book. Besides that, if you want lore, go for Lords of Madness, the Draconomicon, and maybe Libris Mortis and the Fiendish Codex 2, which are a bit more mediocre.
Unrecommendated is Warriors of Heaven, the worst book that was even produced for 2e AD&D. I am also not sure if the Complete books are the right thing for lore; especially the first four are alot heavier on the crunch side versus the fluff side. Especially Complete Psionics I would unrecommend, as it is a subpar book with subpar fluff and subpar mechanics.