Identify your core objective in writing the book. If the real-life individuals you're including in it are all portrayed in a pleasing and flattering light, you really don't have anything to worry about and can, in fact, stop reading this article and get back to work. If, however, your goal is to expose certain named individuals as the duplicitous, rotten, amoral, sadistic, greedy and squirmy weasels you believe them to be, you're courting a potential lawsuit. Proceed to Step 2.
Talk with an attorney. Ideally at this point you'll already have a synopsis and outline of the book to take to the meeting with you. If it's a politically charged tome in which you're lambasting your peers and opponents for getting you ousted from public office, there's a good chance that you're already embroiled in lawsuits. An attorney may suggest that you avoid bringing on any more trouble right now.
Consider what you agreed to when you first started gathering the information. If your book involves material that was acquired during your employment with an entity that had an expectation of confidentiality (i.e., national security, the military, the priesthood, the legal system, the medical community), there's a high likelihood that you signed an oath of silence when you were hired. This can and will be used against you if you proceed. You do, however, have three options available to you if you're still insistent on proceeding. Read on.
Enlist the cooperation of the individuals you want to write about in your book. This will obviously be easier to do if you're saying nice, fluffy things about them. If you're taking a negative slant, you can incorporate interviews in the text that will allow these people to tell their own version of what happened and, accordingly, allow the readership to draw their own conclusions about who was right and wrong.
Write under a pseudonym. The drawback to this approach is that it may take on the tone of anonymous letters. If a person doesn't have the courage to use his own name, he may be perceived by the readership as being someone who is probably lying. You'll also be precluded from including quoted conversations in the text because the other party or parties is likely to read it and say, "Hey, didn't I have this same conversation three years ago with Bob?"
Turn your tale into a fictional novel and change all the names. This happens more often than most people realize, especially in books about romantic betrayals. These not only represent a cathartic outlet for the writers but also serve as an inside joke for all of the writer's friends who know that "Jerry" is really "Lester" and that the snoopy housekeeper "Gwen" who ends up as a chalk outline on the floor was actually "Rhonda," a real-life supervisor who slept with the heroine's husband. The nifty disclaimer at the front of most fiction novels that "the characters, incidents and dialogue are products of the author's imagination and are not to be construed as real" acts as a legal buffer against the real Jerrys and Gwens who might otherwise throw a snit and haul you into court.