A riveting first novel; Out Of The Blue, by James McManus. New York: Crown Publishers Inc. 246 pp. $12. 95.

If possible, make time to read this in one gulp. It's a book you won't want to put down. Jack and Shelley Exley are the parents of five-year-old Elizabeth. They don't have much money; Jack teaches English at the University of Illinois at Chicago, their Hubbard Woods apartment is over a heating-and-plumbing business, they don't even own a car. Out of the blue one sunny October day, Elizabeth is kidnapped, grabbed off the playground by a man and a woman masked in stocking caps.

Why?

In the ensuing turmoil and terror, during which Shelley is taken to the hospital on the brink of a miscarriage, the answer emerges: The kidnappers made a mistake. Their intended victim was Elizabeth's look-alike classmate, the daughter of Burke Rawls, chief executive officer and principal shareholder of NorthCentral Industries, a man who certainly does have money. Undeterred, the kidnappers send him a ransom note demanding a fortune in diamonds for the release of a child not his own.

Will he pay? What is his moral responsibility? Rawls discusses the situation in a meeting with FBI agents, the Hubbard Woods chief of police, and NorthCentral's legal counsel, financial officer, and public relations officer. (These people are introduced to the reader all at once, causing confusion that never gets cleared up.)

Rawls agrees to pay the ransom. Yet tormented Jack isn't sure he will, nor is he sure that the FBI agents will put Elizabeth's safety first. After all, who could trust anybody who talks the way these agents do, assuring him, ''And you definitely will have your input. . . . It's very much a part of the game plan''? Throughout the ''one business day'' allowed by the kidnappers for Rawls to get the diamonds, the nerve-racking waiting continues.

Although there is color in the title of this novel, it is written in monochrome. Details are observed minutely, as when young Elizabeth ties a shoe: ''She makes the right loop first and squeezes it tight at the bottom. She wraps the end of the other lace over this loop, then pushes the middle of the lace back up and through.'' Scenes are set up with stage directions: ''Noon. The empty kitchen of the Exley apartment. Three cane-backed chairs, two metal stools , a small wooden table.'' Conversations are disjointed as are real conversations.

This is a technique which in some books can frustrate the reader, but it suits this story perfectly.