Here's what others have to say about it:
→ book review on wetpixel by Steve Williams
→ customer reviews on amazon.com and amazon.de
UPDATE: And here's my opinion after having read it from A to Z.
Martin Edge's rather heavy 500 pages book isn't quite as encyclopedic as its not-meant-for-traveling weight might suggest, it being more an inspirational guide book to the art of underwater photography than a referential source of dry facts and educational how-tos.
Though not always as detailed or in-depth as I would have liked (*), the text's very easy to read, covering most aspects of underwater photography and always well illustrated with plenty of exemplary photos. (* internet links often point to additional online resources, but on paper these aren't clickable of much help)
I don't know about experienced underwater photographers, but for amateurs wanting to make underwater photos rather than just take snapshots, I'd say Martin Edge's "The Underwater Photographer" is a recommendable book to get started. Without a doubt much more so than e.g. "An Essential Guide To Digital Underwater Photography" (1st edition) by Michael Aw & Mathieu Meur, which I personally found disappointing from every point of view.
