Cora Harrison's latest "mystery of Medieval Ireland" is called "A Secret and Unlawful Killing." As in her previous work, My Lady Judge, Harrison recreates Ireland in the early 1500s, when female Brehon Judges played a central role in Irish life as well as the criminal justice system. Mara (Harrison's main character) must untangle the disparate ties that have led to one murder as well as an apparent suicide. ($24.95 / 336 pages / St. Martins-Minotaur)