Harry Waltz, Loan Shark, at home; Waltz in Marathon, by Charles Dickinson. New York: Alfred A. Knopf Inc. 276 pp.

Most novels are autobiographical in one way or another, to a greater or lesser degree. First novels tend toward pure autobiography, the protagonist a mirror image of the author.

But what have we here? A first novel about Harry Waltz, a 61-year-old loan shark. The dust jacket tells us that its author, Charles Dickinson, was born in 1951 (a date that leaves Dickinson 29 years shy of Waltz's age) and that Dickinson's stories have appeared in many magazines, including The New Yorker and The Atlantic Monthly. It doesn't mention how else Dickinson earns his living , but probably we can take for granted that he is not a loan shark. So the only thing this author seems to have in common with his hero is a life spent in the Midwest.

Harry Waltz is the richest man in Marathon, Mich., and he assumes that eventually he'll also be the oldest, because centenarians run in his family, as do twins. The novel opens at the wake of his 100-year-old father: Waltz has always lived in his father's house, despite having been married, and another of his assumptions is that his life will continue at its quiet pace in this big house on the hill with all the repossessed cars out front. His wife died 20 years ago; his twin daughters are leading their own lives in Ann Arbor; his favorite son was killed in Vietnam; his loan-shark son is in jail, put there by Waltz himself after hearing he was beating up customers and Waltz himself witnessed him robbing a man.

Waltz is a nonviolent loan shark, his manner gentle and reasonable but firm. He enjoys his work. ''I've just always done it,'' he says when queried. ''From about the age of fourteen. I had this magical experience where I lent a dollar to a man in town and the next day he paid me back a dollar and a dime. I asked him what the dime was for and he explained interest to me. I was hooked. It was simple, elegant, straightforward, honorable.''

But now his settled life becomes disrupted. He falls in love with Mary Hale, a 42-year-old lawyer, their romance and its consequences astonishing them both. He learns that the husband of his pregnant daughter is the object of her twin sister's desires. The twins and the out-of-work husband return to Waltz's house for an extended visit. Mary Hale gets his son released from jail; this son also returns home. Meanwhile, the completely unthinkable has happened: His customers have begun to skip their payments to him, and ''The tenets of his life feel shifted and abused.''

''Waltz in Marathon'' is told in the present tense, a technique that can sound affected and cause confusion during switches to past tense for flashbacks. But Dickinson handles the problems as smoothly as possible. This is a startling first novel, mature and professional, abounding with good spirits.