True20 Adventure Roleplaying - Revised edition. Two great books in one package.
True20 Bestiary. Great critters and semi-human monsters throughout, for space, fantasy, historical gaming, horror, or whatever.
True20 Adept's Handbook. Cool stuff with powers for any and every genre.
This is where it gets tougher. From here, it's really a question of what do you really want to do with the system and what do you need books for.
If you want settings, my personal recommendation is Blood Throne. Legends of Exalibur is fantastic for any Arthurian gaming you want to do, but not really necessary, if you understand. The same is true for True20 Wild West; fantastic book but not necessary to play a western with True20. A lot of people love Caliphate Nights, but it just never really appealed to me (I still don't know why, it's very good). I really like Lux Aeternum but many don't; more importantly, the company behind it has ceased supporting the setting, so there won't be any further (almost necessary) exploration of the setting or clarification of its rules. Reign of Discordia looks great but until it comes out no one can be sure. I'm also a big fan of Worlds of Adventure, as it contains a nice selection of settings (though several of them still haven't been further developed).
Nevermore is a freaky little setting that lets you do anything and everything you've ever wanted all at the same time. Crazy dream worlds.
If you wanted tools to use in your own settings then I'd recommend the Vehicles book by the folks that brought us Technothrillers. It's a bit dull to read but it has some great tools for vehicle oriented scenes, adventures, and campaigns.
Blood and Fists True20 is
the book for hand to hand combat. It has a wealth of mechanics, concepts, descriptions and advice that are perfect for any character or campaign that emphasizes marital arts (especially the weaponless kind).
I can't comment on a lot of the more recent products, as support for True20 has increased while my personal time and resources available for it have decreased. My loss.
Good luck and I hope some of this was helpful.