PRESUMED INNOCENT By Scott Turow Farrar Straus Giroux. 431 pp. $18.95

THIS FIRST novel is being billed as the summer's big book, and it's easy enough to see why. Presumed Innocent has all the ingredients of popular success -- suspenseful plot, engaging characters, well-oiled prose -- and it is the beneficiary of a full-bore publicity and advertising campaign. But Presumed Innocent is rather more than just another beach-blanket best seller; it is a book of considerable intelligence and style, one that manages to provoke as well as to entertain.

It is a novel about the law, a subject with which its author is intimately familiar. Not merely is Scott Turow a lawyer, formerly of the U.S. attorney's office in Chicago and now in private practice there; he is also the author of One L, an uncommonly interesting account of his first year at Harvard Law School -- a book that has become something of an underground classic among lawyers and would-be lawyers. In nonfiction and fiction alike, Turow displays both an impressive knowledge of the law and a deep skepticism about its capacity to uncover the truth; it is a revealing, and refreshing, combination.

Presumed Innocent is the story of Rusty Sabich, a 39-year-old prosecuting attorney in a midwestern city that bears a suspicious resemblance to Chicago. He is the number-two man in the office; the number-one man, Raymond Horgan, is up for reelection and is facing his first serious competition in years, from an ambitious former deputy named Nico Della Guardia. Further complicating Horgan's political situation is the unsolved murder of Carolyn Polhemus, a beautiful, ambitious member of his staff who had specialized in rape cases and who for a brief time had been Rusty's lover.

At the outset, then, the ingredients are in place: politics, sex and death. Who could ask for anything more? In Turow's hands, these ingredients are manipulated into a story that becomes ever more complicated and mysterious, but that only occasionally strays beyond the boundaries of credulity. Boiled down to its essence, the story is that Rusty -- a married man, a father, a suburban homeowner, a career man in the prosecutor's office -- finds that his life has been turned inside-out: not merely is his boss defeated, but when the new crew takes over Rusty is charged with, and indicted for, Carolyn's murder.

It is the middle-class citizen's worst nightmare: a good, ordinary life suddenly gone crazily off the track, an innocent man suddenly facing conviction for the worst of all crimes. The legal maneuverings may make Presumed Innocent entertaining and suspenseful, but it is the random injustice of Rusty Sabich's predicament with which the reader really connects. This could happen to any of us: No one has led a perfect life, everyone is suspect in the eyes of the law, and everyone is within reach of the law's proverbial long arm. Further, the workings of the law are neither as rational nor as reliable as we wish them to be; the innocent are as susceptible to conviction as the guilty, and the prisons are open to all.

FOR RUSTY Sabich this injustice is doubly galling: not merely is he an innocent man, he is also a man who has given his adult life to the service of the law, who is possessed by a "bizarre commitment to learning the truth." What he comes to understand, though, is that in certain circumstances the truth is best not learned, and certainly is best not made public. As he is dragged ever deeper into the mire of corruption and complicity that is city politics, Rusty is taught a hard lesson: truth, like beauty, can be relative, and sometimes the most one can hope to settle for is an unsatisfactory measure of it.

Rusty learns this lesson in plausible circumstances and in a setting that is vividly portrayed; Scott Turow has put his own experience on the U.S. attorney's staff to good use, and conveys the full flavor of the courtroom and the prosecutor's office -- the worn, gritty, dirty, impersonal rooms where the public's business is transacted. Further, he is as good at characterization as at description: the dead Carolyn Polhemus, who at first arouses our sympathies but slowly emerges as an implacably malevolent person; Rusty's wife, Barbara, possessed of a "virtual armory of private and largely uncommunicated passions"; Larren Lyttle, the judge, a powerful personality and a man of bottomless complexity; Nico Della Guardia, who even as he prosecutes Rusty "still wants me to like him"; Sandy Stern, attorney for the defense, whose "view of human capacity is itself large enough that I doubt any one act, no matter how heinous, would disqualify someone from his affections" -- and a raft of minor characters, among them cops and crooks and regular people, whose distinct personalities are deftly sketched.

It's a large cast of characters and a complicated story, and for the most part Turow keeps matters moving swiftly and seamlessly. In the first section he uses conversations between Rusty and a psychiatrist to fill in the background about Rusty's relationship with Carolyn; these conversations seem strained at the time they are presented, though a later appearance by the psychiatrist at Rusty's trial lends them a certain ex post facto validity. The novel is also a bit longer than it really ought to be, though in Turow's defense it must be said that readers seem to like some extra flesh in their pop fiction.

But Presumed Innocent is more, and better, than that. It comes to us with no evident literary pretensions, for which we must be grateful, but it is gracefully written and there is a lot more to it than a quick, painless read. Beneath its facile plot is a story about lost innocence: in the courtroom, to be sure, but in the heart as well. Rusty Sabich is not a man who had everything, but he had a good, decent life, and suddenly it is taken away from him. He wants nothing more than to have it back, but life doesn't work that way. So in the end he must take what is left, as all of us must, and make do with it. ::