Three couples - Mark and Celia, Chris and Stephanie, Hope and Leo - live happily in adjacent rowhouses on a small street in Center City. They're all best of friends, an extended family, really, who cook together and watch one another's children. As Stephanie observes, their children are together "every weekend, all weekend, and most weeknights they're either playing outside or in my basement."

A leak that damages the drywall between Mark and Celia's and Chris and Stephanie's homes inspires all three families to take a radical step: They'll tear down the walls between their houses and become one household.

In many ways, the new arrangement (the "kibbutz," as their Israeli contractor, Yogi, affectionately calls it) is what they'd all hoped it would be. Leo, an upscale restaurateur, has a home audience with whom to share his love of cooking. (Meanwhile, endearingly, wife Hope is more inclined to go the Pillsbury cinnamon buns route.) They're all there for one another, only now they're bound more closely.

But these warm and loving impulses take a real battering in this thoughtful and absorbing novel. They discover that the walls had been guardrails protecting against an array of troubles. Hope finds, for example, that her generosity in caring for the three couples' children gets taken advantage of when all the kids come down with a stomach bug and she's left to deal with it alone. Her housemates aren't indifferent to her pain, just preoccupied with competing priorities.

And, with the walls down, sexual temptation is easier to act on, and the privacy to deal quietly with family problems is harder to find. These new difficulties play out most poignantly in Mark and Celia's troubled marriage. They've been drifting apart, and Mark is fighting "that nagging feeling of coming unglued." Celia, successful and a model of organization, knows she's making up for childhood hurts: "Never getting any attention, she had always said, made her self-sufficient, and self-sufficiency was the key to an organized - and happy - life."

Into these two lives of quiet desperation walks Nikki, the new neighbor down the street, with whom Mark starts a passionate affair.

For Mark, the affair seems to bring clarity to his discontent and make the emotional distance from his wife even greater. One night, after their young son's cries put the kibosh on Mark and Celia's lovemaking, "Mark couldn't shake the phrase that kept running through his mind: that was close."

The new arrangement strains Chris and Stephanie's marriage, too. Each is keeping big secrets from the other, the kinds of secrets that can't be kept forever.

Amid all the existential threats are more mundane ones. Because of zoning concerns, and the fear their kids will be stigmatized by this lifestyle, they decide to maintain the public fiction of three separate residences. That means they have to remember to walk in and out of the right door, and they can't invite outsiders in, and that includes their children's friends.

They also have to contend with Mary, their inquisitive neighbor across the street. (Many readers will instantly recognize a neighbor like Mary, who tells Mark at his birthday party: "I remember when I turned forty. . . . That was right about the time I started losing control of my bladder.")

For those who know and love Center City Philadelphia, Pretty Little World is a road map of its neighborhoods and shops. Part of the enjoyment of reading it is to visualize exactly where things take place. Both authors are Philadelphians. Elizabeth LaBan, who is married to Inquirer restaurant critic Craig LaBan, is the author of The Restaurant Critic's Wife and The Tragedy Paper. Melissa DePino is the founder and director of Leapfrog Group, a branding and marketing firm for nonprofits. Their affection for the city is a subtle undercurrent in their novel.

Pulling together a resolution for all the problems of this many characters isn't easy, and the ending of Pretty Little World isn't totally convincing. But it does show respect for the characters and the lessons they learned. And it allows that not all of those lessons were sad.

Rhonda Dickey is a former Inquirer editor and writer.