Mobbed / Carol Higgins Clark.

Enhanced descriptions from Syndetics:

New York Times bestselling author Carol Higgins Clark is a native of New Jersey. So is her sleuth Regan Reilly. In Mobbed, Clark sends Regan "down the shore" where they have both vacationed since childhood. But this trip turns out to be no vacation for Regan!

PI Regan Reilly and her husband, Jack, head of the NYPD Major Case Squad, plan to spend the weekend at the Jersey shore with her parents. Regan's mother, suspense writer Nora Regan Reilly, will be celebrating her birthday on Saturday. But Regan's weekend gets off to an early start when she receives a phone call from her mother asking if she'd hop on a train right away.

Regan had just finished delivering bad news to a high school classmate. Hayley Patton is a successful New York City event planner, who had become suspicious that her beau, Scott, was seeing other women and hired Regan to follow him. When Hayley got the lowdown on Scott's treachery, she vowed revenge with such vehemence that Regan is worried that Hayley might go too far.

Turns out Nora had also just spoken to an agitated high school classmate. Karen Frawley Fulton, who lives in San Diego, called Nora after learning her mother's plans for the day. To Karen's shock, Edna Frawley just sold their home at the Jersey shore and is having a garage sale to end all garage sales. Everything the beautiful young actress Cleo Paradise left behind when she recently rented the house is up for grabs. The irrepressible Edna even rented a plane to fly over the beach touting the sale, using Cleo's name as bait. Karen's heading East but asks Nora to please go to the house and, she hopes, curtail any other outrageous antics her mother probably has in mind. Nora agrees and asks Regan to join her.

Regan and Nora arrive at the Frawley home to find a mob of garage-sale junkies waiting at the front gate. Regan is astonished to see Scott's brand-new fiancée, who quickly makes herself scarce.

Once inside the house, Regan becomes increasingly suspicious about why Cleo left so many belongings behind when she vacated Edna's home unexpectedly. Was she in that much of a hurry? Regan wonders. "Where did Cleo go?" is a question too many people are suddenly asking. But no one has the answer. It isn't long before Regan finds herself searching for Cleo, whose next hours may be her last.

Mobbed is another "fast-paced mystery with unexpected plot twists and Clark's trademark humor" ( The Daily Beast ).