Published in the eighties, and very much of its time - when many people were experimenting with sexual relationships outside marriage or couples. The key relationship in the book is a triangle, between Willie, a sculptor who does construction work to earn money, his wife Susan, a fabric designer, and Dinah, a composer. They have lived on Cape Cod for eleven years, all year round, and their relationship is accepted by the locals.
The book is told from several different points of view, and the chapter headings are simply the name of the PoV person. This annoys me on occasions, especially when it shifts to Susan, the least sympathetic character of the three, and the one who exposes the fault lines in the three-way relationship. She longs for the glitz of New York, as represented by Tyrone the tycoon, one of the 'summer people' of the title.
The them of Jewish identity is explored in the relationship between Dinah and her flute-player lover Iztak.
I may have been aware of more problems in the book than when I first read it about 15 years ago, but I still enjoyed it.