Killers of a Certain Age by Deanna Raybourn

Older women often feel invisible, but sometimes that's their secret weapon.

They've spent their lives as the deadliest assassins in a clandestine international organization, but now that they're sixty years old, four women friends can't just retire – it's kill or be killed in this action-packed thriller by New York Times bestselling and Edgar Award-nominated author Deanna Raybourn.
Billie, Mary Alice, Helen, and Natalie have worked for the Museum, an elite network of assassins, for forty years. Now their talents are considered old-school and no one appreciates what they have to offer in an age that relies more on technology than people skills.

When the foursome is sent on an all-expenses paid vacation to mark their retirement, they are targeted by one of their own. Only the Board, the top-level members of the Museum, can order the termination of field agents, and the women realize...