Straightforward and Journalistic
Hersey primarily sticks to "just the facts, ma'am" style of journalism. Even when he's delving into the experiences of the six individuals who served as his subjects, he appears simply to be reporting on their actions and the events that occurred in their lives in the wake of the atomic blast.
Sometimes, of course, he zooms out a bit from the six subjects and adopts an even more journalistic style, providing a kind of birds-eye view of the Hiroshima aftermath:
Of a hundred and fifty doctors in the city, sixty-five were already dead and most of the rest were wounded. Of 1,789 nurses, 1,654 were dead or too badly hurt to work. In the biggest hospital, that of the Red Cross, only six doctors out of thirty were able to function, and only ten nurses out of more than two hundred. The sole uninjured doctor on the Red Cross Hospital staff was Dr. Sasaki. (2.19)
Hersey veers into this kind of big picture discussion of here and there throughout the book, which definitely adds to the book's journalistic flavor. In the quote above, he actually connects this birds-eye view to his six subjects, "zooming in" by transitioning into a discussion of the crazy challenges Dr. Sasaki would be dealing with as the only uninjured doctor at the Red Cross.
Kind of gives you an idea of why people are so crazy about Hersey's book, huh? His style definitely keeps things objective and historical while also keeping you aware of the human impact of all these events.