It depends entirely on what you are looking for...
If you get the True20 Revised Core Book, you get a toolbox to handle multiple genres, while the original book contained multiple basic Campaign Settings: Caliphate Nights; Lux Aeternum; Mecha vs. Kaiju; Borrowed Time.
The Worlds of Adventure contains 5 more Campaign Settings: Agents of Oblivion; Blood Throne; Land of the Crane; Nevermore; and The Razor in the Apple.
Most of the other True20 source books are either extensions of the core Roles (Adept, Expert and Warrior Guides, which I haven't picked up to date), Genre toolkits (Wild West), or core source conversion/re-envisionings (True20 Bestiary).
Third party companies have taken up producing products specifically with the True20 system in mind, specifically expanding campaign settings from Worlds of Adventure, or adding new resources or campaign settings with a modern or future flair.
Here's a link to True20 systems resources on
RPGNow...
Right now, I haven't needed most of them, so can't give specific recommendations for many of the additional books, as I've been using True20 as the base for an old school Forgotten Realms run with good success.
I'd double check ENWorld, as I know that Reign of Discordia and Interface Zero have gotten a lot of good comments and reviews here from people.