Max Brooks has a zombie war on his hands, and apparently that was terrifying enough: World War Z is an easy, sea-level jaunt across the war-torn beaches littered with the decomposing, undead masses. His sentences ease you in with a tone reminiscent of an easy-going conversation, and his word choice presents a vibe of everyday chitchat.
Difficulties? Well, Brooks does have a love of acronyms and slang. Partly, this is because a majority of his characters work either in the military or government, and people in those occupations just love their work-time slang. But partly this is simply Brooks's own fondness, since he creates acronyms and slang words for his made-up weapons or organizations.
For example, Brooks employs the acronym NRA several times in the novel. This acronym could describe the very real National Rifle Association or Brooks's own zombie war creation the National Reeducation Act (6.1.8). Both exist in the novel's world and sometimes the context doesn't make things perfectly clear. But, like a seaweed-garbed zombie who has washed ashore, these instances can easily be walked around.