The Cat Who Played Post Office is the sixth book in The Cat Who series, published in 1987. Jim Qwilleran has acquired a large mansion, from Aunt Fanny, and of course a mystery. Iris Cobb joins him in Pickax as the mansion's caretaker, along with Koko and Yum Yum. Koko keeps trying to tell Qwill something about the missing housemaid, Daisy Mull.