The Cat Who Played Post Office (The Cat Who… Mysteries, Book 6) - Lilian Jackson Braun

Inheriting unexpected millions has left reporter Jim Qwilleran looking like the cat who swallowed the canary. While his two Siamese cats, Koko and Yum Yum, adjust to being fat cats in an enormous mansion, Qwilleran samples the life-styles of the rich and famous by hiring a staff of eccentric servants. A missing housemaid and a shocking murder soon show him the unsavoury side of the upper crust. But it's Koko's purr-fect propensity for clues amid the caviar and champagne that gives Qwilleran pause to evaluate the most unlikely suspects...before his taste for the good life turns into his last meal.