The books in the Uglies triology are:
Uglies, Pretties, and Specials. For those of you who have not read them. They are set in a future world where citizens are ugly until they turn 16, when they can have a special operation to make them “pretty.”

Extra, according to a review at barnsandnoble.com, is a companion book, set in the same world but with different characters.

 Joyce