Edison's tale of waste and woe emerges, more terrible than any character could have imagined. But the deck is stacked in more ways than one. The book is set in the American heartland of Iowa, where "my fellow citizens are so consistently broad of backside, round of shoulder, stout of leg, and plump of bicep that we might all be trooping across a canvas by Fernando Botero". Even in such a landscape, Edison is so gross that strangers recoil from his smell, his mass, his presence. The pitch of moral revulsion is captured perfectly.

Big Brother is absolutely fearless when it comes to actual bodies inhabited by actual human characters. Sometimes the tone is lyrical: "Manifest to myself in the ethereal privacy of my head, I grow alarmed when presented with evidence of my public body." Other times, it is shockingly, gruesomely animal. There are other novelists who would liken chocolate fudge cake to faecal matter — but only Shriver would dare risk the reader's sympathy by hinging a plot point on actual faecal matter, and pull it off with pathos. Even Irvine Welsh only dared play the subject for laughs.

"What happened to Uncle Edison?"

"I don't know, sweetie."

"Is he sick?"

"According to the latest thinking on the subject" — we heaved to a stand — "yes."

This is Shriver at her cold-eyed best. "One in Three Would Trade Year of Life for Ideal Body" is the Telegraph headline that Shriver has chosen for the book's epigraph. It becomes a chilling forecast as, halfway through the book, Pandora and Edison do just that: devote a year of their lives to an intense crash diet, intent on restoring Edison to a healthy, relatable frame. In the process, little sister becomes watchful, stern Big Brother — and Pandora's own feelings about her marriage, her childhood, her competitive drive, and her loyalties are tested. "When I first took this project on," says Pandora, "I worried it was more than I could handle. But the real project turns out to be much, much bigger. I have to do nothing less than give my big brother a reason for living."

Will they succeed? Here I'll just point out that no contemporary novelist has proven herself less committed to the happy ending than Shriver. But ours is an era of treacle, and not just the edible kind. The antidote is Shriver's psychological, humane, sober prose. In the end, the only word for it is nourishing.