The Other Couple — Louise Candlish

When a millennial couple and a Gen X couple fall into an obsessive friendship, envy turns deadly in this addictive novel of suspense by the international bestselling and award-winning author of Our House.

When Clare befriends a new young woman at work, she and her partner Jamie are living an affluent but predictable life in an elegant house by the river. With fiftieth birthdays not far away, they are ready for new adventures, and Melia and her boyfriend Kit bring a shot of youthful hedonism to their lives. Soon the couples are growing close, with Kit and Jamie even commuting to work together via riverboat.
 
But there is an uncomfortable difference between them: Melia and Kit are deep in debt and obsessed with their struggle to afford the life they feel they deserve--the very life Clare and Jamie seem to be living. Envy and desire simmer dangerously close to the surface.

Then, one morning, Kit doesn't show up at the ferry. When a witness reports that Jamie argued with him the previous night, he becomes the prime suspect in Kit's disappearance. Suddenly, both couples' secrets are spilling out--and a different, darker version of their friendship is shockingly exposed.