I've recently read a trio of thrillers based around infidelity - not with any intent, just the way it went. In all cases the affair at the heart of the novel is between the best friend and the husband of one the main protagonists and each plot explores the consequences which result:

Natalie Daniels: Too Close

In this one, the affair which comes as a devastating shock to Connie, sends her over the edge and with no-one to turn to she spirals into psychosis. The cleverly constructed plot reveals the details of her breakdown as it builds slowly towards the utterly compelling ending where the full, horrible picture is finally revealed.

Lucy Foley: The Hunting Party

It's New Year's Eve and a party of old friends is trapped in a luxurious but isolated Scottish lodge after the snow begins to fall. Secrets are revealed through a multiple narrative and when one of the group ends up dead, the resort manager, who is herself coming to terms with a terrible loss, needs to find out who is responsible as the Police cannot attend until the storm ends.

Claire McGowan: What You Did

When six friends from uni get together for a 25th anniversary weekend, it should be a relaxing break for them all but tensions are evident from the start, then one of the women is brutally raped and things fall apart very quickly as divided loyalties surface and past events are revisited. This one explores some difficult attitudes to domestic and sexual violence as well as the themes of friendship and fidelity which it shares with the others.

All three made gripping reading, completed within a couple of weeks and come highly recommended.