What is the author's tone in Unbroken: A World War II Story of Survival, Resilience, and Redemption by Laura Hillenbrand?

Unbroken is a biographical work of non-fiction that describes Louis Zamperini, its main subject, in a series of dangerous and dehumanizing situations. In the first pages of the preface, which describe Zamperini and his fellow plane crash survivors attacked from above by Japanese bullets while floating on a raft surrounded by sharks, the author introduces a suspenseful tone to the book and introduces questions in the reader's mind about how Zamperini got into that situation and how he survived it.