This is fourth book in the The Uglies series; if you haven't read the first books, starting with Uglies, be warned, this review contains spoilers. Extras can be read and enjoyed without reading the earlier books, although many references will only make sense if you have read the others.
Extras is an extra book; the series was originally planned as a trilogy, and the primary plot is fully wrapped up in the first three books. This does not mean that Extras is not just as good as the others, and if you have read the first three you should certainly read this one too.
This book introduces a new set of characters living in a new city, a city on the other side of the planet, on the island once known as Japan. This doesn't really make much difference, as this city also had the brain-numbing operation, was also freed by the revolution in Specials, and is now facing exactly the same issues as every other city in the world. This city has taken a different path in overhauling their economy, however, choosing to go with a reputation economy. One gets merits for doing things that help the community (i.e., working), but one also gets a 'face rank', a measure of fame. Getting into the top few thousand in the popularity ranks means an automatic life of luxury.
Aya Fuse is fifteen years old, and thus she is still an Ugly. But in the new order this doesn't prevent her from trying to make it big. She is basically what we would call a blogger, producing news stories to post on the web. Each time one of articles gets viewed she gets a bit closer to being famous -- although she has a long way to go.
Her luck starts to change when she catches a glimpse of someone riding on top of the maglev train -- clearly illegal, but new, and clearly a highly exciting undertaking. If she can be the first to break the story it is bound to get some views. So she goes undercover into one of the city's newest and most daring cliques, the Sly Girls. But it turns out that the Sly Girls have discovered something more than just a fun new way to risk their lives, and Aya has to find a way to break one of the biggest stories of the century.
Extras is just as engaging as the first three books, having its own culture and characters that are well-developed and develop well. Towards the middle of the book Tally and Shay, and then David, come to help Aya; this is almost a disappointment, as the story was doing quite well without them. However, it does give us a chance to get a bit more closure on Tally and her friends, which I suspect was part of the reason this book was written.
The biggest downside to the story is that the central mystery is blindingly obvious if you have read much science fiction. Which is not too uncommon when reading children/young adult stories, and is forgivable in the context of a society that does not have science fiction (all the interesting stuff was lost when the old world collapsed). But it is still somewhat frustrating for the reader.
Extras is an excellent addition to the series, but does not have the strong final ending that Specials does. Sadly, it appears that no further books are forthcoming, although there is certainly room left for more books in the future, should Westerfeld ever be inspired to write them.