A true-crime author and historian sent examples of how strangers, via email, feel free to claim his time and work:
“Hi there, I manage the X restaurant in Madison, Wisconsin. Our original owners were gangsters from Chicago. I know you write books about gangsters. Any information you can send us would be appreciated.”
“I’m writing a book about gangsters and cited your work in several chapters. I hope that's okay. Would it be possible to ask you some questions via email?”
"Please send me two free copies of your book."
He simply doesn't answer such requests. It's a sign of maturity if authors can say to themselves, "I don't owe this," if they think they don't. Authors are writers who have successfully protected their time.
I looked up ways other prominent authors and experts handled the availability issue. Some websites claim the author "has no personal email address." Some post an email address whose mail they might open occasionally; others have email addresses for queries related to specific books. Some have message boards or forums for postings, or blogs or Twitter accounts that make them appear more interactive and accessible than they are. There are many ways to handle unwanted requests, if that happens to be a problem.