Hiroshima is a book written by John Hersey. It was first published in 1946 and follows the story of six people who were each lucky enough to survive the dropping of the atomic bomb on Hiroshima on August 6th, 1945.

The part of the sentence that you are quoting in your question actually continues like this: "Each of them counts many small items of chance or volition ... that spared him." Therefore, you could interpret this to say that the author is going to investigate in the book what exactly led to each of these survivors' lucky escape.

To elaborate on this further, you might want to point out that it is estimated that about 66,000 people died as a result of this attack. It is therefore no surprise that the survivors would count themselves to be incredibly lucky to not have fallen victim themselves. You might want to continue to say that it is probably part of human nature to want to know why one was lucky in events such as this.

Therefore, this quote tells us that we can anticipate that the survivors in this book keep going over this fateful day time and time again, looking for clues in their memory to see where they might have made a decision that then eventually led to saving their time. This might have been something seemingly insignificant, such as "a decision to go indoors" or "catching one street car instead of the next."

You could therefore argue that the book teaches the reader the importance of seemingly small decisions in life: every decision we take, as insignificant as it may seem at the time, could potentially have a much bigger impact on our life than we originally anticipated.