Definitely nonfiction. David McCullough has a reputation for presenting factual accounts, and 'The Pioneers' is likely to be based on actual historical events and characters related to pioneers.
Well, to determine if the book is fiction or nonfiction, you need to look at elements like the plot and the nature of the story. If it's imaginative and not based on real life, it's likely fiction. If it's based on actual events, people, or facts, then it's nonfiction.