Laying Down the Law : DEGREE OF GUILT By Richard North Patterson (Alfred A. Knopf: $23; 548 pp.)

In a country beset by the highest crime rate in the Western world, it is little wonder that legal fiction has become something of a cultural pastime. Unless Americans suddenly lay down their semiautomatics or lose their fascination for the courtroom, Richard North Patterson seems destined for celebrity status, alongside Scott Turow and John Grisham, as an acknowledged master of the genre. “Degree of Guilt,” Patterson’s fifth and most ambitious novel, proves that he belongs among the elite.

Like Turow and Grisham, Patterson is a lawyer and an accomplished litigator. Currently a partner in a blue-chip San Francisco law firm, he was formerly an assistant attorney general for the State of Ohio as well as the Security and Exchange Commission’s liaison to the Watergate special prosecutor. In “Degree of Guilt,” Patterson draws upon this impressive and varied background to construct a story rich in realism and suspense.

The action opens with a confusing and sensational event--the shooting of renowned author Mark Ransom by a beautiful TV newswoman, Mary Carelli, in a sumptuous hotel suite. The mystery, unlike that presented in most crime dramas, is not whether Carelli pulled the trigger, but why. Did she act in self-defense, as she told the police, or is she guilty of murder or a lesser degree of homicide?

These thorny questions are thrown into the lap of the book’s protagonist, San Francisco lawyer Christopher Paget, a polished, well-bred and successful specialist in white-collar crime. Fans of Patterson will recognize Paget from the author’s first novel, “The Lasko Tangent,” published in 1979.

Back then, Paget was a 29-year-old Wunderkind with the Economic Crimes Commission, assigned to investigate the illegal funneling of $1.5 million to the President’s reelection campaign by financier William Lasko, one of the Chief Executive’s closest friends. Two key witnesses died during the inquiry and Paget himself narrowly escaped an attempt on his life. When the dust finally settled, Paget’s boss on the ECC and Lasko were sent off to prison and the President’s political career was shattered.

The Lasko affair is not only an interesting reference point for Christopher Paget but also the key to the intricate plot twists that unfold in “Degree of Guilt.” Mary Carelli, it turns out, was an assistant to the imprisoned ECC chairman before she took to journalism, and it was only with her help that Paget was able to convince Congress of the authenticity of his corruption charges.

Mary is also the estranged mother of Paget’s 15-year-old son Carlo. Her lover briefly during the Lasko investigation, Paget has only bitter memories of Mary. Neither he nor Carlo has had any personal contact with her since Paget acquired custody when the boy was 7. Now, she phones with an urgent plea for help after her arrest for the Ransom slaying, which, not surprisingly, has become headline news.

Paget, who has studiously avoided publicity since making the cover of Time magazine for bringing down Lasko, is stunned to hear from her. Against his better judgment, he agrees to take on the case.

Apart from the personal quagmire the decision to represent his son’s mother creates, Paget soon encounters another equally agonizing situation: He thinks his client is lying. In her post-arrest statement to the police, Mary claimed she accidentally shot Ransom at a range of two to three inches during an attempted rape. The problem is that the physical evidence doesn’t support her story. Except for a bruise under one eye, there are no signs of a physical struggle or a sexual encounter, and the entry wound on his body shows that the fatal shot was fired from a distance of several feet or more.

Most peculiar of all is Mary’s assertion that Ransom’s sexual attack was prompted by the playing of an audiotape from a therapy session between the deceased actress Laura Chase, a sex goddess and suicide victim in the mold of Marilyn Monroe, and her psychiatrist.

If the only element of suspense were whether Mary’s improbable defense of attempted rape could somehow succeed, Patterson would have provided more than enough for a taut courtroom thriller. But Patterson shows that he is much more than a legal tactician. Paget’s search for the truth takes us on a grand tour of various people’s pasts, hidden motives and political secrets.

The one weakness in Patterson’s work is that, except for Paget’s able young female associate, his characters at the all-important gut level can be somewhat underwhelming. This is especially true of Paget himself. Although he is a far cry from the cardboard super-heroes of John Grisham’s books, Paget is a character so uncommonly self-contained and aristocratic in bearing that he never utters a foul word, never loses his cool and never experiences the normal swings of human emotion, even in the face of enormously threatening events. It is unclear whether his annoying sang-froid is Patterson’s way of conveying world weariness or reflects a deficiency in the author’s otherwise formidable literary skills. But in the end, it is a defect overcome by a brilliantly engaging story.