A Great Day for the Deadly — Jane Haddam

Fifteen hundred years after St. Patrick drove the snakes from Ireland an Armenian detective is dueling sin in a small upstate New York town. On the eve of St. Patrick's Day festivities, a young nun in training has been murdered. A man of irresolute faith himself, former FBI agent Gregor Demarkian knows enough about religion to know that St. Patrick's Day falls smack in the middle of Lent: a time of penitence.

With hemlock in the body and mysterious apparitions abounding, Demarkian faces his most bizarre case. As more bodies fall and the solution is about to go up in flames, Gregor finds that putting all the clues together in time will take a miracle.