It happens in the Hamptons : a novel / Holly Peterson.

Enhanced descriptions from Syndetics:

In the Hamptons, everyday people are as complicated and fascinating as millionaires...

When Katie Doyle moves across the country to the Hamptons, she is hoping to find summer employment, new friends for her young son, and a chance to explore a new love affair with George, a dazzling investor. What she finds is a strange cocktail of classes, where society's one-percenters vacation alongside local, hardworking people who've lived in the Hamptons for generations. Though she's looking forward to her move, Katie is wary about mingling in her boyfriend's East Coast elite circles. She soon discovers Southampton isn't all that it seems to be on the surface--and neither are the people who live there.

As George takes Katie on a whirlwind tour of country clubs, haute couture, and lavish events, Katie is amazed to witness sudden whims becoming dire needs, extramarital affairs blossoming right and left, and people purchasing friends and loyalties like a pair of shoes. Even the middle-class townspeople maintain determined façades while maneuvering like sharks among the wealthy summer invaders.

The more Katie becomes immersed, the more she learns the stories of both the upstairs and downstairs, the upper crust and middle of the road. The combustion between classes becomes explosive as the summer tears on. Betrayals, a sexual predator, and a missing person lost in murky waves drive the reader on a racing Learjet through impossible twists and turns before landing at the shocking conclusion. When Katie meets Luke, a marine biologist and teacher, he makes her realize what it is she really wants, as she understands the life she's begun for herself is built on shifting Hamptons' dunes.