Spell Compendium, Magic Item Compendium, Unearthed Arcana, PHB II, DMG II, MM II & III, Eberron Campaign Setting, Magic of Eberron.
I haven't decided whether or not I'm going to buy Spell Compendium and Magic Item Compendium. I'm likely to buy the former but I'm not sure about the latter. I'm not into Eberron, so I've got no real use for those two books. I have all the others, however.
I recommend MM V. Skip MM IV, however, unless you can get it dirt cheap.
Other books to consider if you can't find the aforementioned or you have more spending money than you thought and actually bought all the aforementioned and still have some left over: Citsycape, Dungeonscape, Frostburn, Stormwrack, Sandstorm, Planar Handbook, Complete Warrior, Complete Adventurer, Complete Divine, Complete Arcane & Oriental Adventures (don't forget to email me if you buy this one, I'll reply back and send you the Dragon Magazine 3.5 update).
I have all those too.
Rev, can you please send ME the v.3.5 update for Oriental Adventures? I never got a copy of that Dragon Magazine.
3rd Party Books: Any of the Relics & Rituals, Nyambe, pretty Much anything Green Ronin, Arcane Evolved, Transcendence, Gary Gygax's World Builder, Traps & Treachery I & II and Legends & Lairs: Sorcery & Steam. Alternately I highly reccomend purchasing The War of the Burning Sky print products through Lulu, you won't regret it.
Clay_More,
Personally, I think the Relics & Rituals books are okay. They didn't grab me, however. (I only own the Excalibur one.) Creature Collection is an awesome book, however. If you can find the 3.5 version, buy it. Period.
Green Ronin's books are all good, even the v.3.0 stuff. Bastards & Bloodlines (v.3.0, I think) is a twisted book but in a fun way. Your campaign world will never be the same. (Anything Freeport! Nuff said.) Hammer & Helm (v.3.0) is also good. Advanced Bestiary (v.3.5) is great if you like templates. And Monsters of the Mind (v.3.5) has some of the most unique psionic monsters in it out there. If you want an alternate, grittier game system then try True20. (It rocks!)
I'm a big fan of the Legends & Lairs series of books. Spells and Spellcraft (v.3.0) is one of the best arcane-based books I've ever owned. The Seafarer's Handbook (v.3.0) is good but D&D Stormwrack is better. I recently bought Portals & Planes (v.3.5) and it is now one of my favorite books in the series. The Path books are very hit & miss, however. Read those very carefully before you buy any of them. I only own Path of the Sword (v.3.0). Mythic Races (v.3.0) is excellent. Dungeoncraft (v.3.5) is okay (good fluff) but D&D Dungeonscape has better crunch.
For a good rogue book, go with The Book of Roguish Luck (v.3.5) from Malhavoc Press. I also recommend Anger of Angels (v.3.5), Beyond Countless Doorways (v.3.5), The Complete Book of Eldritch Might (v.3.5), Hyperconscious (v.3.5), and Requiem for a God (v.3.5 if you get the PDF version, I think the print version is only v.3.0) from Malhavoc.
From Necromancer Games, if you want a great city setting, then get City State of the Invincible Overlord (v.3.5). It's good, even if you don't use it for the Wilderlands campaign setting. Bard's Gate (v.3.5) and Gary Gygax's Necropolis (v.3.0) are good. Glades of Death (v.3.5) and Ancient Kingdoms: Mesopotamia (v.3.5) are really good. Tome of Horrors Revised (v.3.5) is excellent but it's a PDF only!
Anyway, those are my suggestions!