Pantheon. 291 pp. $29.95

In marriage, wrote the British novelist Enid Bagnold, "Each is familiar with that ancient child in the other who may erupt again."

Or, as in Joshua Henkin's exquisite tale of a couple who meet in college, marry, then forge ahead together for nearly two decades, that ancient adolescent. And undergraduate. And twentysomething. And . . .

Matrimony makes a wonderful title for this beautiful second novel by Henkin, the Philadelphia author of Swimming Across the Hudson (1997), and son of the great human rights scholar Louis Henkin.

Like his father, albeit in a different genre, Joshua Henkin understands what matters to people at their core. He's a writer who knows that the delicate reality of a loving, imperfect marriage over decades poses more enduring truths than a potpourri of sensational plot twists or flashy false exotica.

The title Marriage would have sounded too generic, too blunt a branding for a subtle narrative. Matrimony rightly suggests antique stateliness and nobility, bold enterprise suddenly ritualized, with all the complications that implies.

Thoughtful, literary Julian Wainwright and beautiful, beguiling Mia Mendelsohn meet in a laundry room in 1986 during their first semester at Graymont College in Massachusetts, an unconventional liberal arts college that resembles Hampshire.

He's the WASP son of a Sutton Place investment banker who'd like Julian to follow him into finance. Julian, however, managed to get John Cheever's autograph on the steps of the 92d Street Y when he was 13. He plans to be a writer

Mia grew up Jewish in Montreal, the daughter of a McGill physics professor and a rising academic mother turned faculty spouse. As a teenager, she experienced an Orthodox phase, pressuring her parents to keep kosher at home. Now she's gravitating toward psychology.

Both are also friends with Carter Heinz, who, last name notwithstanding, is a middle-class sort from Sausalito, Calif., son of a ne'er-do-well businessman. Also an aspiring novelist, Heinz, forced to clean dormitory bathrooms to get through Graymont, completes the human triangle that evolves into a prism through which each will see their lives and self-identities.

Yes, they sound like characters you know, and perhaps have met under different names, in different college towns. But you probably liked and cared about those characters, as you'll like and care about Julian, Mia and Carter. Henkin lets you watch adult life sneak up on them as he limns their telling moments.

The Graymont years of Matrimony, with the electric presence of professor Stephen Chesterfield, 57-year-old novelist manque and showboat teacher of fiction, make a delightful set piece.

Chesterfield unloads 117 maxims on his charges over a semester, including, "THOU SHALT POPULATE YOUR STORIES WITH HOMO SAPIENS," and "THOU SHALT NOT UTTER THE PHRASE, 'SHOW, DON'T TELL' WHEN DISCUSSING ONE ANOTHER'S SHORT STORIES."

Anyone who uses "workshop" as a verb gets bounced from Chesterfield's class. Anyone who "uses semicolons correctly" gets an A. His dictum is that "you should write what you know about what you don't know, or what you don't know about what you know."

Matrimony rises to a must-read for writers and lovers of literary fiction because, along with its deft and frequently funny insights into marriage, class and love, it's a novel about how we write - and teach the writing of - literature today.

When Mia's mother becomes ill with breast cancer - the two's interactions form another of Henkin's fine sequences rich in character - Julian and Mia marry at the end of senior year. They move on to the University of Michigan, where Mia pursues a doctorate in psychology and Julian teaches freshman comp and works on a novel. Later, he also attends the Iowa Writers Workshop.

At the latter, where Julian's fiction undergoes some savaging, we hear the following about his work, a mission statement that might apply to Henkin's as well:

"The story was quiet; all his work was. Perhaps it was a matter of differing aesthetics. There had emerged in American fiction a strain of excess, he believed, a group of knowing authors whose every sentence seemed to shout, 'Look how smart I am.'

"He had nothing against muscular prose; it was the flexing of those muscles that he objected to, and, along with it, a disregard for character, which, for him, was what fiction was about."

Yet such observations on writing never weaken the book's fundamental concern with its title theme.

Frustration, separation, monotony, uncertainty - all rear their ugly heads. Mia once slept with Carter, something Julian learns only much later. Carter, who married Pilar, his own Graymont girlfriend, and became a dot-com millionaire, later divorces her.

By the end, we're still with Julian and Mia, and they're still with each other, in Manhattan, with a new home. They've hit the heights, and seen the depths, been through the ups and downs. They've traded off many things - places, preferences, priorities. They've been back to Graymont for their 15th reunion.

He's still writing, with reasonable success. She's a psychotherapist treating post-traumatic stress disorder, and dealing with the discovery that she has the "Ashkenazi" gene for breast cancer. Richard, their 6-month-old, is enjoying rides on Cooper, who's becoming more horse than dog.

Till death do them part? Who knows? What's important is that they're still together. Thanks to Joshua Henkin's artistry, and grasp of the sweetness of life, we understand why.