An Insider's Guide to the World of Uglies.
Or, in other words, THIS IS A RATHER SPOILERIFFIC BOOK; DON'T READ IT IF YOU HAVEN'T READ THE Uglies-Pretties-Specials-Extras SERIES YET. If you have no intention of EVER reading the series, though, you can go ahead and read this book; it's interesting even if you don't know the literary context.
It provides background information on the technology, the science, the history and the societal structure of the world of the Uglies/Pretties/Specials/Extras. AND includes maps. Which is nice, since it helps clarify and explain the (future) world in which the series took place, which wasn't clearly done in the series.
It is written for a teenaged reader's reading level, which makes sense considering the Uglies series is YA fiction; I found it rather vague on details in the chapters dealing with science, but that's okay.
According to my library, this book is a 813.6 on the Dewey Decimal Classification; it is a non-fiction book on American literature in English, about fiction. Well look at that.