PRIVATE SCREENING. By Richard North Patterson. Villard. 322 pp. $16.95.

RICHARD NORTH Patterson holds the enviable distinction of succeeding in two careers. As a lawyer, he was part of the heady action in the Watergate Special Prosecutor's office. As a mystery writer, he scored a first-book triumph with his Edgar-winning novel, The Lasko Tangent (1979). His second novel, The Outside Man, was even better. If a third, Escape the Night, belabored a tired Freudian theme, his latest marks a rousing return to form.

A measure of Private Screening's merit is that one can guess its outcome early on and still find it a compelling read. In large part this is because at the book's center Patterson has placed a long set-piece: a depiction of trial strategy so enthralling and convincing that it shouldcorporated into law-school curricula.

In the dock sits Harry Carson, a woebegone Vietnam vet who assassinated a presidential candidate during an appearance at a fund-raiser with his rock-star girlfriend, in front of 20,000 people. As if that many eyewitnesses weren't enough, two cameras recorded the event from different angles. The thankless task of defending Carson falls to Tony Lord, a struggling, ambitious San Francisco lawyer.

As Lord tries to establish the only plausible defense -- insanity -- Patterson shows the reader a cagey trial lawyer at work. Lord uses up his last peremptory challenge to the inclusion of potential jurors on the panel by vetoing "a female executive who kept smiling in his direction; hard experience had taught him that smilers voted against the lawyer they smiled at." At critical moments in a cross-examination, Lord will make a point of moving between the witness and the prosecuting attorney to cut off facial communication between them. In the judge's chambers, Lord strikes just the right note of respectful direness -- no judge relishes the prospect of being reversed on appeal -- to elicit more favorable rulings for his client.

Made privy to these maneuvers, all but the most cynical readers will find themselves rooting for Lord to succeed in getting the creep off. At the same time both Lord and Stacy Tarrant, the rock star, are such appealing characters that their being on opposite sides of the case makes for a tugging tension. One hopes against hope that the lawyer will win both an acquittal and Stacy. A year after the trial someone using the name Phoenix kidnaps one of Stacy's friends and her business manager. Among the ransom demands are that Stacy give a special concert and that the friend's husband donate $5 million to the poor. Once these conditions are met, Phoenix will conduct a national TV referendum on his victims' fates. Lord's strength and shrewdness had so impressed Stacy during the trial that she asks him to advise her in this crisis. He agrees, and one thing leads -- as it often does -- to another, which is fine because Lord's wife is a shrew.

The trial and kidnapping have more in common than the leading man and lady. For one, Carson the assassin played a similar role in Vietnam, as a principal hit-man in Operation Phoenix. The manifestations of Carson's post-Vietnam syndrome -- flat voice, bursts of rage, amnesia -- seem exactly right, but then so does almost every other bit of behavior in the book. For example, when he first got home from the war, Carson used to arrange crumpled paper on the floor around his bed so he could detect his parents' approach and stop crying. (There isn't a lot of sense to this, but that is part of Patterson's point.) When the candidate is gunned down, his last words are eerily perfect: "Such a joke. But what does it mean?"

The action culminates, somewhat implausibly, in a high-sunset showdown between Lord and Phoenix. But the setting -- the remote, rugged King Range coast of Northern California -- and Patterson's way with interpersonal mayhem serve as lubricants for any last-minute creakiness in the plot. Even after a winner emerges, Patterson has a few lesser surprises in store for anyone smug about having figured out the greater one.

I think Patterson is on his way to being the best mystery writer we have, almost on a par with Britain's great Ruth Rendell. Private Screening is a book that enlarges one's acquaintance with humanity while also satisfying the burning itch to see what happens next. Give it a private reading.