The Ghost and the Haunted Mansion (Haunted Bookshop Mystery #5)

Bookshop owner Penelope Thornton-McClure didn't believe in ghosts, until she met the ghost of hard-boiled 1940s private investigator Jack Shepard. And when Pen's friend and mailman, Seymour Tarnish, gets into deep trouble, Pen not only believes in her ghost, she also thinks he can help...
And elderly lady of leisure has been found dead on posh Larchmont Avenue, her will recently, and suspiciously, revised to name Seymour as heir to the mansion. Just as eyes turn to him as the murderer, and Seymour gets busy settling into his ritzy digs, the mansion's ghosts begin plaguing him.
So he hires a team of parapsychologists to exorcise all the spirits from the town of Quindicott, and that includes Jack. Pen must act fast, because losing Jack scares Pen more than rattling chains and cold spots...