The Bishop and the Missing L Train by Andrew M. Greeley
The Vatican has just assigned auxiliary Bishop Gus Quill to the Archdiocese of Chicago over the violent protests of Archbishop Cronin, and the not-so-silent protests of Bishop Ryan. Bishop Quill is under the illusion, one might say delusion, that he has been sent from Rome to replace the good Archbishop when in fact Rome was dying to get rid of him because of his incompetence. Immediately on arriving in Chicago, he manages to disappear while riding the L Train, and it is up to Blackie to find him. As Archbishop Cronin says, "The Vatican does not like to lose bishops, even auxiliaries."
And thus begins the search for the missing bishop who no one really wants to find.
Of course, none of this is too much for the intrepid little Bishop Ryan. He faces these problems squarely and, with the kind of deductive mind reminiscent of G. K. Chesterton's Father Brown, manages to find solutions to some of the most baffling mysteries he has ever encountered.