Elinor and Ted Mackey have been unsuccessfully trying to have a baby for awhile, and the strain on their marriage leaves Ted open to an affair with Gina, the personal trainer at his gym. Elinor's discovery of the affair sets a series of breakups, reconciliations, and realignments of relationships.
Despite the overly dramatic plot summary, Winston conveys the story with humor and humanity, as she shifts viewpoints among the three main characters. This is the second consecutive book I've read in which infidelity is a plot point, and I think she does very well in illustrating how complex it can really become. Having been in the midst of a similar situation, I was impressed that each character was capable of evoking my sympathy and some degree of understanding. I've also discovered that as I've grown older, I've become far more tolerant of novels with ambiguous endings - that's much more realistic than neatly tying everything up, after all.
I liked Winston's previous novel, Good Grief, much more than I expected to, and was optimistic about this one as a result; however, I liked Happiness Sold Separately both less than I'd expected and less than the earlier book. It's a fairly quick and non-objectionable read, but I don't find myself with a lot to say about it.