One of the reasons so many people hate lawyers is because they sign their names to frivolous, sure-to-be-tossed-outta-court lawsuits or letters threatening frivolous, sure-to-be-tossed-outta-court lawsuits. One assumes that when Donald Trump told his to sue Michael Wolff over the new book, the attorneys explained, "There's close to zero chance of this suit prevailing. All you're going to do is drive up sales on that book."
To which Trump responded, "I don't care. I want that bastard sued!" so they sent a letter threatening to sue. Trump has that "If someone hits me, I hit back" mentality and that — and the bully pulpit he's made out of his Twitter feed — are the only two ways he knows of hitting back. He may also have thought the threat of a lawsuit against the book would dissuade others from writing similar books and if he did, I'll bet he was wrong about that, too. I mean, we've all seen how Trump's suit against Bill Maher has stopped comedians from ridiculing Donald Trump.
Here is the letter sent by Trump's attorneys. The lawyers for Wolff and his publisher have responded to the threatening letter and you can read their reply here. I especially like how Trump's lawyers say he will sue in New York over "false light invasion of privacy" and Wolff's barristers point out that New York doesn't recognize lawsuits over that.