I guess you could call this a companion book to the Uglies trilogy. It introduces a new main character although several characters from the first three books do make appearances, including Tally.
I really enjoyed this book. While the trilogy was good on its own, I think this book was an excellent edition to the series. If you've read the other books, I'd definitely recommend reading this one as well. If you haven't read the other books, then you should go read Uglies. While this book has a different main character, it does take place after the other three books, and it's important to know what happened in those books before reading this one.
I really enjoyed the whole series, and I'm glad I read them. They're probably my favorite series that takes place in the future after the Hunger Games. I'm reading Bogus to Bubbly (which is a guide to the world) now, and Scott Westerfeld really created an interesting world. I definitely recommend this series to anyone.