A Great Day for the Deadly originally published in 1992 and republished as an e-book by Open Road Media.
I've read one other book featuring Gregor Demarkian, a rather unusual protagonist. In this novel, he has retired from the FBI, but still engages in certain cases that either catch his interest or which he is asked to pursue by someone he knows and respects.
When a young postulant is murdered, the Archbishop of Colchester, John Cardinal O'Bannion requests Demarkian's aid in solving the murder. Of course, one murder then turns into two, and Demarkian must figure out who would want to kill both a young nun and a banker. What connection could there be? And will there be another murder?
Mystery. originally publ. 1992, republ. March 5, 2013.