That pretty much tells you what the book is about. It is entirely presented as a work of non-fiction, purporting to tell the story of humankind's struggle against the effects of a "zombie plague" that breaks out in China (and perhaps other places) and quickly spreads world wide. The book tales off from the ideas set out in the satrical "Zombie Survial Guide" by the same author, but is done "dead" seriously, and quite well.
Once you accept the premise of the mysterious zombie disease, everything else is worked out with rigorous logic therefrom. (The origin of the zombie virus and how it actually works in the body, or how zombies actually function, are topics still "under investigation" at the time of writing, according to the book.) Brooks, the author of the Survival Guide which was supposedly helpful during the great outbreak, was allegedly commissioned by the United Nations to author a formal report on the disaster: the personal reminisences he compliled are 'edited out' of the formal report, and he is given leave to edit and publish them as his own book. "World War Z" is the supposed result.
The book is a fascinating study. Imagine reading Terkel's "The Good War" if you had never heard of World War 2 and had no familiarity with the history of it. "World War Z" manages to both sustain the illusion that he is writing for an audience who survived and were profoundly affected by the events described, and to supply enough information for the reader to piece together the sequence of horrific events. Although the tales told in the book are horrible enough, the convention that they are being told often by ordinary citizens and soldiers who are reticent about going into "gory detail" helps keep the gruesomeness level of the book more in line with an early Steven King thriller than the movie gross-out fests that some "zombie" movies have become. (George Romero is specifically acknowleged, along with Terkel and Hackett, though--.)
The progress of the zombie outrbreak, and the probable responses by governments and individuals are in general very well worked out, although there are some improbable bits, such as Japan's spiritual savior being an elderly gardener who had been blinded by the Hiroshima bomb blast. The strategic and tactical difficulties of fighting a zombie horde are convincingly detailed, although I have to say that I think the ultimate solution, at least as described as used by the United States, has some holes. There is no quick or magical solution, which seems realistic, and the fact that goverments would fall is certainly believable, although there's little idea given of how day-to-day society would have changed.
All in all, a very interesting and creative work if you are at all interested in future histories, military what-ifs, or the current popularity of zombies in popular culture. If none of the above, there's no reason to pick it up.