The Atonement

The third and last book of Kiersten Modglin's Arrangement trilogy.

The first book was Peter and Ainsley Greenburg's attempt to save a failing marriage by seeing other people. The second was the repair after that came apart. This one is what the repair cost.

Ainsley's most recent effort has left the marriage in worse shape than at any point before it, and she is now being played against by a man whose behaviour she can no longer predict. Modglin puts her on a track of escalating paranoia where the only tool she has is her own read on Peter, which the previous two books have spent a considerable amount of time demonstrating is unreliable.

Not a standalone. Start with The Arrangement.