Can nonfiction books have opinions

Yes, but they usually base their opinions on information from a credible source. For example; the author may present information on scientific study, and provide an opinion on the methods used, by comparing them to a similar study.

Many nonfiction authors add their own interpretations, or theory of events, to their books, and may not always have solid evidence to backup their theory. You see this more with history books, where the events are not always clear, and there are hundreds of theories about what actually happened. If read any book regarding the assassination of president John F Kennedy, you will find a lot of facts about the assassination, as well as the author's personal thoughts and opinions of what really happened.