Being a parent is always full of ups and downs, of course, but being the parent of an autistic child, Carolyn Parkhurst writes in her splendid new novel, is a lot like riding a roller coaster that never stops to let you off and catch your breath. Sometimes you marvel at your child’s brilliance. Often, though, you are exhausted by her behavior. You may be prone, like Alexandra Hammond in Parkhurst’s “Harmony,” to compare your growing helplessness to the proliferation of bedbugs in your house.

“It’s like a metaphor come to life, your home polluted with invaders you can’t even see. And what if -- you suppose this is the real source of anxiety for most people -- what if the invasion goes even deeper than that? You’ve been to the fringe parenting websites and the homeopathy section at Whole Foods. You know that there are people out there who will tell you that it’s too late, that our bodies are already tainted. That we’re overrun with mucus or bacteria or spreading fungal growth. ... Has depression ever been this widespread, or autism or infertility or food allergies? Something’s changed, even if it’s just our own method of record-keeping.”

In “Harmony,” the Hammonds -- Alexandra, her husband Josh, and their two daughters, Iris and Tilly, the fiery star around which the family satellites revolve -- leave their home in Washington, D.C., for a remote communal camp in New Hampshire, and to Parkhurst’s credit you never question their wisdom in making the move. Their desperation is real, their actions evidence of their need for hope.

Tilly, 13, has been diagnosed with an unspecified “pervasive developmental disorder,” a “diagnosis of exclusion, nothing more than the doctors throwing up their hands and saying ‘Something’s going on here, but we can’t say exactly what.’”

Like many children on the autism spectrum, Tilly displays genius-level intelligence and creativity; on a shopping trip to buy a birthday gift for a cousin, she suggests “a machine that sings songs to you when you’re sad. It knows when you’re sad, because it has eye-recognition technology, and it can see when there’s a tear.” But she struggles picking up on social cues, is prone to obsessive behavior, unable to fit herself into adult expectations (one school asks her to leave because she can’t stop licking the walls). Her parents don’t know what to do.

Then one night at a Chinese restaurant, Alexandra spots a flier on the bulletin board. “Do You Have a Challenging Kid?” it asks. Does she! It’s her introduction to Scott Bean, who’s seeking families with children on the autism spectrum to join him in a communal living experiment called Harmony. The community will be self-sustaining; guest families will arrive each week for a seven-day stay. But the Hammonds and two other families, plus Scott, will be full-timers, forging a new, hopeful, better future. Most people would be skeptical, and the Hammonds are. But need outweighs caution.

A mother of two with a son on the autism spectrum, Parkhurst is especially deft at examining the complexities facing parents of autistic children. She doesn’t resurrect the disproven vaccines-are-the-cause argument, but she makes clear how frustrated parents, even educated ones, might grasp at any explanation for what has changed their lives so drastically. The lack of answers weighs heavy on them. “There are too many people with the same story for it not to mean anything, although it may not have anything to do with mercury or thimerosal or the MMR vaccine. It may be years before anyone finally figures out what’s going on, but there’s something there. It’s going to come to light eventually. And who knows what it’ll be?”

The story is narrated mostly by Alexandra, who chronicles the events that drove the Hammonds to Harmony, and Iris, 11, who writes about life at the compound (which she mostly likes, despite the lack of iPhones and iPads). Iris is smart and observant, quicker than the distracted adults to pick up on Scott’s odd inconsistencies. Tilly also weighs in enigmatically from time to time, adding to the book’s growing sense of dread.

Parkhurst has created memorable characters before, particularly in “The Dogs of Babel,” about a grieving widow/er/ who decides to teach his dog how to communicate, but here she truly excels at bringing Alexandra and Iris to life, her terrific prose matched by compassion and a sense of humor. She nails Iris’ complicated love for Tilly as well as her flashes of pettiness -- Iris knows she’ll never get the attention her sister gets, and she‘s not always happy about that. Parkhurst also understands every moment of Alexandra’s anguish and fear and delight and how weathering the demands of society and her family drive her to seek escape -- in computer games, online erotica, even alcohol.

Also the author of the novels “Lost and Found” and “The Nobodies Album,” Parkhurst as has always been an engaging and thoughtful writer, but the beautifully written “Harmony” is her best work, a haunting, creepy but ultimately moving story of love and family. “Some days you’re an idiot,” Alexandra thinks to herself, “and some days you’re a f _ _ _ idiot.” Most times, though, Parkhurst’s Hammonds are nothing more than thrillingly human. (TNS)

“Harmony”

By Carolyn Parkhurst

Pamela Dorman Books/Viking (278 pages, $27)