This one won't come as a surprise to anyone who's read the novel—or any of our learning guide: World War Z is written like a compilation of interviews.
As a result, the writing style is very chatty, with lots of slang. Almost like you're having a conversation with another person. Here's an example of what we're talking about:
Everything had kind of a retro feel about it. Our Lobos looked like something out of, I don't know, Lord of the Rings. Standard orders were to use it only when necessary, but, trust me, we made it necessary a lot. (8.2.3)
Todd Wainio's whole attitude is just shooting the breeze: the slang term retro, the offhanded reference to Lord of the Rings, the comical emphasis on a lot. Also notice how the sentences have little interjections within them—the "I don't know" and "Trust me" located between the commas. It all adds up to something more in line with a conversation than a doctoral dissertation.
Brooks's writing style grants us access to the zombie war at its most personal level. Of course, if you like your zombies with a little less extraneous detail, then Max Brooks has you covered with his Zombie Survival Guide.