Title: Nine times nine
Description: Before he became a famous mystery novel and sci fi critic, Anthony Boucher wrote this locked room mystery under the penname H. H. Holmes. Voted by critics as the ninth best locked room novel of all time, it concerns an ultra-Catholic family, one member of whom was investigating a religious cult.  Though most of the investigation is handled by a young journalist and the stalwart cops, the impossible murder is solved by Boucher's continuing character Sister Ursula, a nun who grew up wanting to be a policewoman.
Sister Ursula unmasks the criminal using her religious knowledge. The novel in one way is a tribute to John Dickson Carr whose Hollow Man and its analysis of locked room possibilities are frequently referenced. 
Besides frequently quoting Shakespeare, devout Catholic Boucher contrives scenes to make Catholic customs and beliefs less mysterious to non-Catholics.
Subjects: Anthony Boucher, John Dickson Carr, Catholic detective, nun detective, locked room classic, Cults, Fiction, Monasticism and religious orders for women, Murder, Investigation
Places: Los Angeles
People: Sister Ursula
Time periods: 1940's