BOOK REVIEW : Model Mayor’s Obsession Ends in Murder : BAD DESIRE <i> by Gary Devon</i> , Random House, $18.95, 326 pages

Henry Slater would be a shoe-in for any public office he chose. Young, energetic, articulate, independently wealthy and married to a charming and public-spirited woman, he seems wasted as mayor of a small California city.

From all appearances, he should have his eye on the White House, or at least Sacramento. Why would a man of Slater’s talents settle for grappling with parking zoning ordinances when he could be making laws for everybody?

Gary Devon answers this question on Page 13 of “Bad Desire,” after you’ve seen this paragon of civic virtue meet another man in a seedy bar far from the pleasant plazas of Rio del Palmos.

There, in a dim back booth, the two conspirators write terse messages to each other in spilled sugar, tipping the canister onto the table, then sweeping the grains away for the next line. They never speak, but by the time they leave, money has changed hands and you know something horrid will happen before midnight Saturday.

Henry Slater is insanely in love with Sheila Bonner, a 17-year-old high school junior. They met when she was a child of 10, living with her grandmother, Rachel Buchanan, after her mother’s death.

Slater watched the pretty child grow to luscious adolescence. Though the Slaters now have a lavish estate on the opposite side of town, he hasn’t lost touch. The mayor continually cruises in one of his three cars, trailing Sheila from school to cafe to mall, spying on her day and night.

So far, Sheila has been accommodating, thrilled to have a secret admirer and delighted with his gifts of jewelry. Unfortunately, these treasures stay hidden in her closet. Her grandmother doesn’t approve of the relationship, and has forbidden her to see Slater alone. The meetings are furtive and increasingly unsatisfactory for him. She now has a boyfriend, a football star acceptable to her grandmother, and she’s beginning to tire of the dangerous game with the mayor. The boyfriend has made Slater not only impatient, but desperate. If the grandmother could only be eliminated, he thinks, all obstacles to his union with Sheila would melt away.

The crucial difference between a mystery and a psychological thriller is that in the latter you know the villain at once. The excitement must come from watching him enmesh himself ever more deeply in evil as the clues accumulate and suspicion heightens.

In “Bad Desire,” neither the police chief nor the betrayed wife act upon their knowledge, effectively depriving the novel of the villain’s only antagonists. Without them, tension dissipates like the morning fog over Rio del Palmos.

Given his druthers, Slater would have preferred the murder of Rachel Buchanan to be a neat shotgun job by the hired assassin. But when the hit man is killed in a bizarre accident before he can fulfill his assignment, Slater takes matters into his own hands.

Rachel Buchanan is amateurishly but gruesomely killed, and, as Slater had hoped, Sheila turns to him for the consolation he’s so eager to provide. The trinkets escalate into major gifts; the brief encounters stretch into passionate afternoons in Henry’s secret house, a remote property he bought under a false name for just this purpose.

We soon learn that Slater is an expert builder of sophisticated explosive devices. From his cushy office in City Hall, he detonates these all over town, then calls emergency council meetings to deal with the crisis.

Coincidentally, a trio of escaped convicts has been terrorizing the coast, distracting the police chief and remaining at large long enough for Slater to complete his metamorphosis from model citizen into serial killer.

For a few weeks, Slater enjoys his idyll with Sheila, all the while keeping up a busy social schedule with his wife and working overtime at his mayoral duties.

But honeymoons eventually end--some gradually, others abruptly. Slater’s is the second kind, and when it’s over, it turns out that Henry Slater isn’t the only person in Rio del Palmos to suffer from sexual obsession. But this one won’t come as any surprise, either.

Next: Carolyn See reviews “Possession: A Romance” by A.S. Byatt (Random House).