Hersey writes in a very readable journalistic style, taking the facts (as presented to him by the six survivors, presumably) and weaving them into coherent sequential narrative of the day's events.
It could have gotten a teensy bit chaotic, given the difficulty of keeping track of six different stories—and the fact that their stories intersect in multiple places—but the prose and "plotting" are smooth and totally easy to follow. In fact, the most confusing thing in the book is the fact that two unrelated individuals have the last name.
Hersey keeps the literary bells and whistles to a minimum, with his journo style letting the facts do the talking with little editorializing on his part. The result is that the horror of the bomb drop and its aftermath come across loud and clear, without a lot of frills or melodrama—trust us, reality offers more than enough drama in this tale.