Specials, by Scott Westerfeld, finished up the Uglies trilogy with a bang. Tally Youngblood has been transformed once again and is now an agent of Special Circumstances, the super-fast, super-strong, and super-ruthless secret police of her future city.
Holy Cow, was this book good! The first 50 pages are one of the best action sequences I've ever read in a book, and it doesn't let up from there. And there are surprises and plot twists and... wow.