A Nun in the Closet — Dorothy Gilman

When Sister John and Sister Hyacinthe set out to explore the huge old house left to The Abbey by a mysterious benefactor, they had no inkling of the bizarre adventures that awaited them.

From the moment they set foot on the property, their world began to shiver and shake. First they found a wounded man hiding in a closet. Then they discovered a fortune stuffed in an old suitcase at the bottom of the well. There was also the matter of white powder in the sugar jars in the pantry. Sugar? Not on your life.

Gangsters, ghosts, young rebels--you name it, they found it. The nuns were up to their wimples in trouble. They were also developing a lot of strange habits.