Something Borrowed by Emily Giffin.
I received this book through the GoodReads first reads program.
It's the story of Rachel White, a young attorney in Manhattan. Darcy, her best friend since childhood, is engaged to Dexter, who went to law school with Rachel. Rachel thinks Darcy is tall, slim, beautiful and charismatic, always feels mousy and insecure next to her, and has resigned herself to being in Darcy's shadow. Darcy is really loud, brash and arrogant, and treats Rachel almost like Cinderella.
The story begins on Rachel's thirtieth birthday. A group including Rachel, Dexter and Darcy is bar-hopping. Darcy overdoes it and goes home, while Dexter and Rachel go to "one more place". Rachel is surprised when Dexter ends up spending the night with her. She and Dexter then have to figure out how to keep their tryst from Darcy, who, they are certain, would react very badly were she to find out.
The remainder of the book follows the friends through the summer as they rent a beach house together. Rachel tries to be a dutiful maid of honor, at the same time as she is wracked with guilt.
Under normal circumstances, I would be horrified by Rachel and Dexter's behavior. But Darcy is an incredibly arrogant prima donna, and frankly, I identify with Rachel, having been the "mousy friend" most of my life.
The ending completely surprised me, although it probably shouldn't have. Giffin is a pretty good writer and I enjoyed the book more than I thought I would.