X had a 1994 retrospective at the Museum of Modern Art; she wrote seminal novels under various pseudonyms; one of her scripts was filmed by Wim Wenders; she produced records for Tom Waits and David Bowie (and wrote the lyrics to “Heroes”). She discovered and recorded a singer who resembles Karen Dalton. She corresponded with Denis Johnson and was photographed by Annie Leibovitz; she crashed Andy Warhol’s parties and spurned Warren Beatty’s advances. She was everything everywhere all at once. She would never use a door if a window were available.

By late 1996, X is dead. The biography that emerges a year later, by a man named Theodore Smith, infuriates C.M. It’s lightweight and literal, and it’s a joy to watch C.M. attack it. She calls it “radiant with inanity.” She says it reads as if Smith “has mixed up a palette of pastels and given himself permission to brighten a Rembrandt.” She notes that he gets crucial facts wrong.

This is a magpie novel, one that borrows snatches of text, that tinkers with reputations, that moves historical figures around in time. When C.M. writes that Smith’s biography is “page by page, line by line, without interruption, worthless,” some readers will recognize these words, altered just slightly, from Adler’s 1980 takedown, in The New York Review of Books, of Pauline Kael. I’m on the Kael side of this divide, and this repurposing, linking Kael with a hack biographer, rubbed me the wrong way, but that’s life, and it’s nit-picking, and it’s a whole other freeway.

C.M. sets out, in her grief, to report her own biography, a project she refers to as “a wrong turn taken and followed.” Her reporting takes her out into an America that is recognizable, but barely. Like Philip Roth’s “The Plot Against America,” this is a mighty work of counterfactual history.

There is room here only to sketch the outlines of the world that Lacey convincingly projects onto the page. The country was divided, in the “Great Disunion of 1945,” into Northern and Southern Territories, and a wall was constructed between them. The South has become a tyrannical theocracy: Women wear long dresses, the radio plays only church hymns. Lacey employs photographs to ghostly, Sebaldian effect. One image is a satellite photograph of America at night, in which the Southern Territory is completely dark; it’s like looking at a nighttime image of North and South Korea. Lacey spoons out the horror:

On that autumn day in 1945, the quiet orderliness began. Phone lines were snipped. Radio stations were shut down — some by violence and executions, others by willing consent. Local newspaper production ceased. Electricity and running water were rationed in the small number of homes that had any to begin with. Sunday church attendance became mandatory. Libraries were purged of unlawful texts. Schoolhouses were abandoned — all education took place in churches now. Armed guards stood attention at the few places where it was possible to cross the border; snipers were stationed along the rest of the wall. No one was allowed in or out, and those who dared to defy these orders were shot dead.

Lacey, whose previous novels include “Nobody Is Ever Missing” and “The Answers,” has long been interested in characters who grew up in religion-deranged families or were otherwise off the grid. We learn that X grew up in the Southern Territory — born Caroline Luanna Walker, in 1945 — and that she was a rare escapee.

Diane is admitted to Orchard Springs, an enormous hospital that appears to have been dropped onto its parklike campus “without any apparent plan.” One might say the same of “Commitment,” which has a meandering, aimless vibe until around Page 75. Simpson lingers for a bewilderingly long time on the minutiae of Walter’s life, then dips briefly into Lina’s (she’s 16, a junior on the honors track at Pali High, a school Diane got her kids into using the address of a woman she met at exercise class). Donnie, the youngest, is twice neglected — first by his mother, then by Simpson, who mostly ignores him until much later in the book.

But once Diane is in the care of a decent doctor, the path of “Commitment” becomes clear: It’s a survival story. Walter, Lina and Donnie will have to figure out how to take care of themselves. Sometimes they’ll be OK; sometimes they’ll flounder. Occasionally they’ll function as a team, but mostly they’ll adopt a solar system model, orbiting the sun (Diane, no matter how long she’s absent from their daily lives) while being steadied on their axes by Julie, who is the moon. A cynical reader might find Julie’s selflessness too convenient; I found it inspiring and wanted to know more about her. Instead I learned a lot about Thomas Story Kirkbride, the Quaker psychiatrist who believed that airy, well-lit hospitals could have a curative effect on patients. He was interesting too.

Simpson seems to have unlimited time and pages as she follows Walter, Lina and Donnie into adulthood, through graduations and first loves and soul-crushing jobs, from Los Angeles to New York City, into the realms of architecture and art and parenthood. Walter and Lina build their adult lives around the creation and destruction of beauty, as if the chance to exert control over a sculpture or a building might make up for the unsteady foundation of their family life. Simpson has clearly done her research on the development of the Pacific Palisades and on the gallery scene in Manhattan in the 1980s, among many other topics, and the fruits of her labor add texture to an already hefty story.

Donnie’s trajectory is less obvious than those of his siblings. He floats where the wind takes him; “trouble became his natural habitat,” Simpson tells us. Of course, “everyone in high school had found out what happened to his mother. He’d never told, but they knew. Girls wanted to talk about it, their voices pitying, hands eager.” When Donnie’s drug addiction becomes too big to ignore, the Azizes finally have to do the work they’ve avoided for so long. The therapy-speak is mine; Simpson would never be so heavy-handed. Her language is subtle to the point of coyness, with an arm’s-length quality that’s equal parts impressive and maddening.