A vow of adoration / Veronica Black.

Enhanced descriptions from Syndetics:

With her friends Brother Cuthbert and Detective Sergeant Alan Mill both away, Sister Joan of the Order of the Daughters of Compassion must rely on her own wits when she finds the body of a homeless man in a creepy abandoned chapel high on the Cornish Moors near her convent. The man's identity remains a mystery, as do the activities of Michael Peter, owner of an upscale antique shop and waxwork show and husband of a woman named Crystal. When Crystal's sister comes to Sister Joan for help, the unconventional nun must use all her skills to discover the truth, but before she can do so another body is found. In the ninth adventure in the series of delightful whodunits featuring everyone's favorite nun-sleuth, Sister Joan must, in the course of her investigations, learn the difference between adoration and infatuation, and risk her own life in the process.