The Pact by Jennifer Sturman

From the publisher (source: The Internet Book List):
A mystery for anyone who has ever hated a friend's boyfriend...

Rachel Benjamin and her friends aren't looking forward to Emma's wedding. The groom is a rat, and nobody can understand what Emma sees in him. So when he turns up dead in the morning of the ceremony, no one in the wedding party is all that upset. Not even Emma.

Rachel, who had the good fortune to find Richard floating facedown in the pool, is feeling as if she's woken up in an Agatha Christie novel. It doesn't help that everyone around her seems to have a motive for murder. So, while the cops detain Emma's family and friends at her isolated Adirondacks compound for the weekend, Rachel, an investment banker by trade, makes like Miss Marple (minus the gray hair and sensible shoes) and does some digging of her own. Her investigation gets especially tricky when Peter Forrest, the too-good-to-be-true best man, turns out to be the number-one love interest and her number-one suspect. And Rachel can't help remembering the solemn pact she and her friends made back in college -- a promise to rescue each other from bad relationships, using any means required. Has someone taken the pact too far?

This book is part of the "Rachel Benjamin" series.
Here are some other books from this series: