Henry
Autoexreginated
Flexor, I'd probably recommend AGAINST the Spell Compendium, given that you are just starting back up in 3.5. Your spellcasters would have a field day, but this one single book may have been most responsible for me getting "mechanics-tired" in our last Eberron game.
I do recommend the PHB2, as others have, because the four classes are IMO well-balanced with the PHB classes, the spells are flavorful without any really tricky mechanics, the feats give high-level fighters a well needed extra punch above 11th level, and the affiliations and retraining rules in the back are very good for getting players more involved with the campaign setting itself. I don't know if you'd need the retraining rules if you use something similar on the fly, but they're a good set of rules for players who regret a choice made earlier, and want to change up without dumping their existing character.
Past that, if you're starting slow "core only", I can't think of any that your game would really suffer without.
I do recommend the PHB2, as others have, because the four classes are IMO well-balanced with the PHB classes, the spells are flavorful without any really tricky mechanics, the feats give high-level fighters a well needed extra punch above 11th level, and the affiliations and retraining rules in the back are very good for getting players more involved with the campaign setting itself. I don't know if you'd need the retraining rules if you use something similar on the fly, but they're a good set of rules for players who regret a choice made earlier, and want to change up without dumping their existing character.
Past that, if you're starting slow "core only", I can't think of any that your game would really suffer without.