Yep, PHBII is full of wonderful character-building goodness. It will be the most useful of just about any book you could want.

Beyond that, it just depends on what sort of game you want to run. The Races of X books are best for new races, especially the setting-specific ones. The Complete X, likewise, are best for new classes and feats.

Tome of Magic has some variant casters that are fairly nice, from what I understand it, but you may want to hold off on it for something that will give you a bit more bang for your buck (especially since it takes some work to get the full use out of this book).

You may consider getting the Expanded Psionics Handbook. Most of it is in the SRD, but you might find it easier getting used to the system with the book there in front of you.

I wouldn't bother with Unearthed Arcana. Not that it isn't useful, mind. But most of the useful information is in the SRD, so you can safely spend your money elsewhere.

And don't hesitate to use these forums as a resource as well. We have a number of homebrew monters, races, and classes that you might find useful (especially anything by Vorpal Tribble).