The Postmortal is set in our near future after a cure for aging is developed. Doesn't mean you can't die, mind you -- you can still be shot, or fall of a bridge, or get cancer -- but you won't get any older and you won't die of old age.
Magary does a fine job of exploring the possible social and political fallout of such a cure. There are new religious cults to deal with; new legal structures surrounding marriage and divorce are developed (not to mention the redefinitions of "life sentence"); pregnancy becomes taboo (because with fewer people dying, the world's resources are strained as it is); and inevitably, a new government bureaucracy arises to deal with the postmortal who decide they've lived long enough and are ready to die.
The best thing about the book is that it does a lot of things well. It's a smart extrapolation of one idea and its consequences; it's a creepy apocalyptic thriller; and it is very funny, in a dry and dark way. And Magary builds the story well; the consequences of living in a post-cure world don't all appear at once, but instead build up and develop over time. Things are worse after forty years than they are after ten, in ways that are entirely sensible and logical.
If there's a serious flaw, it may be that the narrator doesn't seem to mature much as the book progresses; the 89-year-old at the end of the book feels about the same as the 29-year-old who gets the cure at the beginning. You could, I suppose, argue that aging is an integral part of maturing, and that if the physical self doesn't age, neither will the emotional self. But even that potential problem didn't really strike me as I was reading the book, which I enjoyed as a surprisingly smart piece of light entertainment.