The Holy City Murders by Ron Plante Jr

Duke Dempsey is a private investigator in 1938 Charleston, SC. Thrust into the case of a lifetime when the Vatican comes calling and hires Duke to find an invaluable relic. Charleston is also hit with the mysterious murders of a prominent priest of St. Patrick's Cathedral and a local cop. Duke is forced into a partnership with the presiding detective, Johnny Stampkin, to find the relic and solve the murders. The case takes a multitude of twists and turns as they navigate through Roman Catholic puzzles, Civil War secrets, and a Nazi Assassin.