FOUR AND TWENTY BLACKBIRDS by MERCEDES LACKEY softcover

A magical murderer is loose in Alanda. The victims are always women, always lower-class, and the weapon is always a three-sided stiletto, most often found among Church regalia. But the killers are never churchmen, and they always commit suicide immediately after the bloody deed. Tal Rufen is just a simple constable. But he really cares about his job, and when one of these murder/suicides happens on his beat he becomes obsessed. His superiors don't care - the victims will never be missed, and their murderers are already justly dead. But every instinct Tal Rufen has cries out that he has seen only one small piece of a bigger and much nastier puzzle....