The title World War Z is a rather blatant riff on the name of two rather famous 20th century conflicts, World War I and World War II. Perhaps you've heard of them? Obviously, the title plays off the idea that the entire world is entrenched in this war, just like the previous world wars. Only this time, there are, you know, zombies.
But what about the full title, World War Z: An Oral History of the Zombie War? That subtitle is also an allusion, only a tad less obvious one.
Back in the ancient days of yore—1984 to be specific—a writer named Studs Terkel put out a Pulitzer Prize-winning book titled The Good War: An Oral History of World War Two. In this nonfiction work, Terkel interviewed people around the world to get their firsthand stories of the Second World War, starting form Pearl Harbor and going all the way to end. Sound familiar? Yep, Brooks fashioned his fictionalized account of the zombie war after Terkel's very real account of World War II (Source).
So, we have a title doubling down on its duties. On the one hand, the title tells you exactly what you'll be reading on the pages between its covers—that is, a world war plus zombies. On the other hand, the title alludes to its inspirations, hinting at all the themes and ideas that any literary history brings with it.
Oh, and in case you don't want to take our word for it, here's Max Brooks talking about his title choice in an interview. Always good to have an extra opinion or two.