The short answer to this question is yes, at least if the person in question is living, and especially if you portray them in a negative light. Avoiding legal problems isn’t as simple as changing a person’s name, either. If someone else figures out who you’re talking about, even if you’ve changed their name in your book, you could still face a lawsuit. For best results, Writer’s Digest recommends that you: “Disguise the names and biographical data and make sure that no one can identify the subjects from your description. Use a pseudonym if need be. And ALWAYS (it’s in all caps for a reason) talk with a knowledgeable lawyer first.”