Birthmarked is about Gaia, a terrific female protagonist, who has a burn scar on her face, and thus, has been "unwanted" by the Enclave that she serves. She is very useful, however, as she has been trained by her mother as a midwife. Soon, she realizes things are not as they seem, when her parents are arrested for treason. She then tries to rescue them, and encounters adventure and danger at a breakneck speed. 
 I loved so many aspects of this book. First, Gaia, is a wonderful main character, which is so important since we see this world through her eyes. She is not whiny, and she is strong, resourceful and intelligent. Also, because of her burn, she is not perfect looking and has to survive in a world where appearances are very important. I thought the setup of the world was fascinating. Although, I do wonder about siblings intermarrying without knowing it, and why the Powers that Be didn't think of this issue. I also love all the puzzles that are in the book, and how Gaia figures them out. Lastly, I think it's interesting how her burn serves as both protection and as a mark of shame at the same time. 
 Overall, interesting characters, exciting plot, and O'Brien has marked herself as an author to watch.