The Spy Coast by Tess Gerritsen

 This appears to be the start of a new series featuring a group of retired CIA agents who have settled in a small town in Maine. They are happily living their new lives when a woman named Bianca shows up at Maggie Bird's house and tell her she has to help a fellow former agent named Diana, who is missing. Since Maggie blames Diana for the greatest tragedy in her life, she has no interest in helping her. Then Bianca turns up dead in Maggie's driveway. And Maggie realizes this may be tied to her last case at the Agency. And she might need the help of her fellow former agents to untangle everything.

It's quite the page-turner. And it reminds me more than a little of the Thursday Murder Club series by Richard Osman. I look forward to the next adventure if this is indeed the beginning of a new series.