Because Max Brooks writes World War Z: An Oral History of the Zombie War in anecdotal form (which means as a collection of stories), it is difficult to say that one person is the protagonist. Brooks seems to be the central figure—the cohesion—in the story: the person that holds the "report" together: he is the one constant. He might be considered the protagonist because he is continually present in the novel except for the definition below.
Dr. L. Kip Wheeler defines a protagonist as:
the main character in a work, on whom the author focuses most of the narrative attention
Brooks' accounts are based on interviews that he has conducted that describe the spread of the zombie infestation throughout the world. I do not believe that the "narrative attention" focuses on Brooks as much as on those telling their stories in the interviews. In this case, each story, or section, has a different protagonist.
Much like Chaucer in The Canterbury Tales, while he includes himself as a member of the pilgrimage described in the Prologue, Chaucer provides the structural framework (through the use of the pilgrimage) to bring a wide and diverse group of people together. In this way, Brooks uses the structure of the interviews to collect a wide variety of stories about World War Z. In my opinion, Brooks is not the protagonist, but each interview or story has its own.