Agatha Raisin and the Haunted House (Agatha Raisin #14) by M.C. Beaton

Just back from an extended stay in London, Agatha Raisin finds herself greeted by torrential rains and an old, familiar feeling of boredom. When her handsome new neighbor, Paul Chatterton, shows up on her doorstep, she tries her best to ignore his obvious charms, but his sparkling black eyes and the promise of adventure soon lure her into another investigation.

Paul has heard rumors about Agatha's reputation as the Cotswold village sleuth and wastes no time offering their services to the crotchety owner of a haunted house. Whispers, footsteps, and a cold white mist are plaguing Mrs. Witherspoon, but the police have failed to come up with any leads, supernatural or otherwise. The neighbors think it's all a desperate ploy for attention, but Paul and Agatha are sure something more devious is going on. Someone's playing tricks on Mrs. Witherspoon, and when she turns up dead under suspicious circumstances, Agatha finds herself caught up in another baffling murder mystery.