BOOK REVIEW / NOVEL : When You Don’t Know When to Trust : DAYS OF DRUMS by Philip Shelby; Simon and Schuster $22, 320 pages

First things first: This is a successful thriller. L.A.-based Philip Shelby’s tale of a rookie U.S. Secret Service agent’s encounters with a psychopathic killer and with corruption at ever higher levels of government is intricately but clearly plotted, taut and exciting.

From the beginning, when the killer, nicknamed Preacher, waits in duck blind and plans the knife-torture and assassination of a U.S. senator, until the end, when bullets are ricocheting around the Oval Office, “Days of Drums” engages the cogs in our nervous systems that keep our wrists and elbows turning the pages.

The rookie agent, Holland Tylo, is haunted by the murder of her father, also a senator. When she makes a minor error that lets the diabolically clever Preacher skin Sen. Charles Westbourne alive, it means not only the end of her career but also the frustration of her lifelong dream of public service.

Disgraced and put on leave pending her firing, Tylo discovers that the outwardly admirable Westbourne has been gathering dirt on his colleagues for years and blackmailing them to further his presidential ambitions. By mistake, he gave Tylo a computer diskette containing half of these damning revelations. It must have been what the killer was after. And who has the other diskette?

Before long, Tylo has more than her reputation to save. Her life is threatened and several innocent bystanders are butchered in a series of chases around Washington that let Shelby display his knowledge of Secret Service procedure and weaponry.

Tylo’s problem isn’t just that Preacher and his shadowy employers want her dead; she no longer knows whom to trust. The group of powerful senators called the Cardinals who shared Westbourne’s last supper? The head of the Senate Intelligence Committee? The head of the Secret Service?

Even the President--a fan of University of Arkansas football who is referred to only by his code name, Razorback--may be implicated in the Westbourne diaries.

In an age, supposedly, of rampant ego, Shelby’s publishers, and no doubt many of his readers, will appreciate his selfless subordination to the demands of the genre.

Take Preacher, for instance. If he were a zealot of some kind, Shelby would have to lead us into the swamps of political partisanship, but of course Preacher is no such thing. He’s a thriller killer--purely evil. “It’s [his] nature,” a superior tells Tylo. “He wasn’t made into what he is. He just is.”

So, too, with Westbourne and the politicians he has been blackmailing. One conspired with the Soviets to hide a spy scandal resembling the Aldrich Ames case. Another oversaw biological warfare experiments that killed 15,000 people in Arizona nursing homes.

Why? Did they have a mistaken sense of Cold War priorities? Were they seeking to advance any recognizable political program? Did they have any human qualms?

No. They were thriller politicians--totally rotten. They did bad things because they had the power; because their rottenness feeds (and feeds off of) the public cynicism about politics that Shelby is exploiting; and because the contrast between Washington’s noble architecture and the slime balls who work there today highlights Tylo’s disillusionment.

It’s just their nature.

So, too, with Agent Tylo. We sympathize with her and admire her sturdy feminism, but she also represents the self-pitying Praetorian Guard attitude that thrillers promote. If elected officials are crooked, the public ignorant and the media easily duped, virtue resides in the unelected, underpaid soldiers, cops and spooks who serve the nation more faithfully than it deserves.

“In my body,” Tylo’s boss tells her before blowing his brains out, “I carry bullets meant for men who are so much less than you or I.” Says another agent, “Guys like [the President]--they only rent space.”

“In the end,” Tylo concurs, “Razorback understood very little about those who protected him, the things they held sacred, the loyalty to each other that was the crucible of their creed.”

This is the attitude that leads to military coups in Third World countries. Not that Shelby means to encourage anything of the kind.

He’s just writing a thriller. And thrillers just are.