This book, as the title suggests, is written by a couple of guys who are from the sort of background where they've come into contact with the emergent church - and they explain not only why they are not emergent but also why they believe emergent thinking to be potentially dangerous to the gospel.
The authors take it in turns to speak, with authorship of chapters alternating between them. This gives two very different styles and 'takes', which is quite refreshing - DeYoung is a pastor and his chapters are more theological; Kluck is a journalist and writes more from his own experiences. The book never goes particularly deep, so although there are chapters of theological 'stuff', it's not overly hard to understand.
The book doesn't spend too much time examining what the emerging church is - as it explains, it's difficult to pin that down and even the leaders of the movement have resisted doing so. Instead, the authors generally take quotes from emergent literature and discuss the strengths and flaws that they see in it.
I've never read most of the books they talk about (e.g. Velvet Elvis, Blue Like Jazz etc.), so I can't say whether it's a fair representation of what those say or not, but a lot of what they mentioned as the potential failings of the movement did strike a chord with me as I thought about friends who would describe themselves as emergent. I do think that to get a balanced viewpoint, it would be a good idea to read some emergent-type writing, but this is definitely a useful introduction to the more questionable aspects of some of what the movement has produced.
One thing I didn't like so much about the book (and why I gave it 4 stars and not 5 - would've given 4.5 if I could though) was that at times the tone felt a bit 'snarky'; they seemed to be poking fun at the emergent leaders rather than engaging with its teachings. This wasn't overbearing, but it did come through a few times and seemed a bit harsh.
Overall, a useful introduction to the emerging church and to the potential dangers of wholeheartedly embracing it; if you're really interested in finding out in depth stuff you'd probably need to find a more detailed discussion, but this is a good starting point. It would probably also be fair, as I said above, to balance this account with a more 'pro-emergent' one.