BOOK REVIEW : Tension and Crises on Road to Adulthood : SAM’S CROSSING <i> by Tommy Hays</i> ; Atheneum; $19; 255 pages

“Sam’s Crossing” is about growing up in America today. America today being what it is--partly marked by youth fantasies left over from the ‘60s and ‘70s--the person who does the growing up is in his early 30s.

Sam lives in Atlanta where he came years before to write the great American novel. His first attempt ran 100 pages; his second, 50, his third, 20. Dried up, he is reduced to taking an occasional stab at “the great American sentence.” He works in a bookstore. “If he couldn’t write books at least he could work in their proximity. He liked to hold them.”

Sam lives on the periphery but is reasonably content; particularly because he loves the woman he lives with. Kate is not content, nor does she live on a periphery. She works hard and devotedly as a social counselor in a hospital neonatal unit, and she wants life to mean something. Specifically, she wants marriage and children.

Tommy Hays’ first novel tells how, through various crises, advances and backslidings--and with a lot of help--Sam comes to make the kind of commitment that signals adulthood. It is a kindly story, written with wit and a fine sense of its generation and of a setting that has not been much described: a young, racially mixed urban neighborhood in the New South. It is too kindly, sometimes; we are not quite hustled along its upbeat course, but we are unmistakably wafted.

Told as a contemporary comedy of manners, “Sam’s Crossing” takes us through scenes of understated tension between Sam and Kate, softened by their concern for each other. There is Kate’s departure, her affair with a doctor and her eventual return, pregnant. There is a Halloween party and a tornado that first disrupts it and then turns it into a mild orgy, and Sam’s mild-orgy fling with a woman named Grace.

There is a holdup at the bookstore and a medical emergency. Sam copes decently with both. These crises pierce his apathy; his ability to deal with them without disaster builds up his confidence and brings him to life.

There is a pat-ness to the story. As with Hollywood’s rueful-romantic sophistications, “they lived happily ever after” seems to precede “once upon a time.” Everyone is unusually nice; not just Sam and Kate, but their married friends with one baby and another on the way, and a black neighbor who is a nurse and a single mother, and Sam’s tough-love luncheonette waitress who is also a single mother and preparing to put two children through college. All of these, all too explicitly, provide the lesson that life can and must be lived.

The niceness spreads further. Even the holdup man is moderately nice. So is the wife of an old man Kate hits with her car, and so, after a little grizzling, is the old man. So is Grace, who obligingly moves out of Sam’s life and into the life of Willy, Kate’s brother. So is the doctor who will not marry Kate and who wants her to have an abortion; it is because he is black and fears pain for her and the unborn child.

If all this is rather slick, Hays writes with real distinction as he develops the relationship between Sam and Kate. Sam is vague and amiable, and Kate is vivid and cross; and the scenes in which they move apart have a dramatic liveliness that is part comedy and part pain. Hays is particularly good at deflections; the sudden leaps that a dialogue takes when anguish churns at it from underneath.

When they return from visiting a pregnant friend, Kate predicts glumly that the mother’s medical history will produce complications. Clearly, there is more at stake, and when Sam says he’s sure it will be all right, she flares up:

“You just want it to be okay so I can be okay so you can be okay.” Next morning, as she is doing up her blouse, she bursts into tears. “It’s all wrong,” she weeps. Properly alerted by now, Sam shows instant sensitivity. “You and me?” he asks. “I’ve got it buttoned all wrong,” she deflects. A moment later, she announces she’s leaving.

The book’s best passages deal with pain, in fact. There is a wonderfully sustained sequence after a friend’s baby, whom Sam is watching, falls down stairs and goes into a coma. Sam carries it to the nurse next door, she gets it breathing and they rush it to the hospital. The tension over what will happen, Sam’s anguish and guilt, and his shame at confronting the baby’s parents are compellingly set out.

As Sam and Kate begin to move toward each other again, the writing works while their movements are gingerly. With all-out reconciliation, it falters. Hays’ buoyant wit and sympathy show up best upon a somber or at least a contained background; rosiness melts them.