Yes—nonfiction books are written about real events and facts, not invented stories. Cambridge Dictionary defines non-fiction as writing that is about real events and facts, rather than stories that have been invented. Common nonfiction categories include biography, history, science, memoir, how-to guides, and reference works.
That does not mean every claim in a nonfiction book is automatically true: authors can err, interpret evidence differently, or mix in opinion. The genre label means the work presents itself as grounded in reality, unlike novels and other fiction.