The Friends We Keep by Jane Green

Evvie, Topher, and Maggie become inseparable best friends at college in England in the 1980s. After graduation, they go their separate ways--with Evvie and Topher ending up in NYC and Maggie staying in England and marrying her longtime crush, Ben. They try to stay in touch but slowly drift apart until their 30-year class reunion, when they reconnect and realize their strong bond is still there. But then a long-held secret comes out, which threatens to tear apart the newly revived friendship.

Jane Green tells a good story, so this is a fine choice for a waning-days-of-summer read. The characters are all likeable, despite their flaws. And you really hope they can find a way to work things out.