If you wou want some premade adventure sites then I'd reccomend Castle Sites, City Sites & Country Sites
Only have Country Sites, but there's some fun material in there, and it's all plug-and-play.
Two must haves imho are the
Complete Book of Necromancers (awesome book, includes mini island campaign), and Monster Mythology (the non-setting-specific core D&D gods).
Necromancers is a very excellent book. Has lots of new spells, magic items, and kits for both necromancers and priests how serve dark gods. The mini necromancer campaign (set in Al-Qadim) is very well done and contributes to the whole overall flavor of the book. Some great NPCs for the setting that use stuff from the book, with two other NPCs for a Realms campaign. Has the best necromancer spell list outside the Wizard's Spell Compendium itself, with spells from the PHB, ToM, Wizard's Handbook, the original FR hardback, the Al-Qadim setting book, and Sha'ir's Handbook. However, I understand the book is hard to find, plus it's intended as DM only. I'd say good for campaign building and developing villains, but not as much for general use.
Monster Mythology is ok, but I suspect Demihuman Deitites covers the gods most likely to be used by PCs more thoroughly (but with the massive overpower of the series). The DM can probably skip it unless he really wants to use the standard non-human gods.
My take on the splats:
Fighter's Handbook: Best used if you're not using Combat and Tactics. Both books cover similar combat options, but do things a bit differently. The weapon list in Fighter's Handbook is unnecessary with C&T's list. Kits are mostly the generic early 2e stuff.
Thief's: Add a bunch of useful NWPs, and has good rules for thief guild generation. Some additional useful thief gear. Once again, kits are a tad bland, but overall a splat worth using.
Priest's: Way underpowered specialty priests. I found this book more useful as a DM than a player.
Wizard's: Best part are the spells. Some good breakdowns on the different schools. More generic kits You can probably live without this one too if you use the Spell Compendium.
Psionics: Needed if you want to use Psionics, but completely optional. Not a bad approach to psionics as a whole, but psionicist build can be very powerful compared to caster if min-maxed.
Bard: Great stuff for the bard player, and the first book that goes beyond the generic bland early kits. Also allows bards for races that can't take the class under the PHB rules, but with specific kit restrictions.
Dwarves: Best race splat. That's really all that needs to be said. Also useful for the DM with the stronghold creation rules.
Elves: This should have been called the Complete Book of Power Creep. Well, Elves really isn't all that bad unless you hate the whole "elves are better than j00" attitude. Even the Bladesinger isn't a bad idea, but the DM should probably tone it down a notch. Best for the subrace character creation rules.
Gnome and Halflings: Bland. That's about all, this book really doesn't help convince players that these two races aren't boring.
Humanoids: Good if you want the common humanoid races as PCs in the game, also helpful for the DM when creating NPCs.
Paladin's: Doesn't seem too bad, but then I didn't get much use out of this one. This book requires a DM and player who are willing to work together in the game; otherwise, a rule-stomping DM who wants paladins on a tight alignment leash is not good for the book.
Druid's: Great for the terrain variations. Some good material for fleshing out campaign and characters.
Ranger's: A lot like the Druid's book, once again has terrain variations, and sometimes the two books feel a lot alike.