Commonwealth by Ann Patchett

The Cousins and the Keatings are two California families forever intertwined and permanently shattered by infidelity. Bert Cousins leaves his wife for Beverly Keating, leaving her to raise four children on her own. Beverly, with two children of her own, leaves her husband for Bert. The six children involved are forced to forge a childhood bond based on the combined disappointment in their parents. As adults, they find their families' stories revealed in a way they couldn't possibly expect. Read a sample.