I know that he's now busy being the world's most respected zombie expert, and I seem to remember him saying that he had already told the full story of World War Z (and I personally don't see how anyone could write a sequel), but has Max Brooks ever said that he is writing/will eventually write/could conceivably write more books about zombies?
Max Brooks has repeatedly denied that he's currently writing another Zombies book, stating that he's lost interest in the genre.
Are you kidding? Everybody's still trying to get me to do Space Balls 2: The Search for More Money. To be honest, I'm a shitty businessman. If I were better, I'd be on World War Z Part 7. But unfortunately, I have to write the things I'm obsessing about. - GQ
That being said, he's not ruled it out for the future...
Any plans for a sequel [To World War Z]?
Max Brooks: Maybe, I'll have to give it some time and see what germinates. It's always too tempting to rush to capitalize on the success of a previous work and I'm going to have to be VERY careful to make sure another zombie book doesn't suck. - Washington Post
The cynic in me says that as soon as he writes something that flops, he'll immediately put pen to paper on a new zombies novel.