The Fourth Crow by Pat McIntosh

Maister Gil Cunningham is Blacader's quaestor, the man in medieval Glasgow charged with investigating suspicious deaths and bringing the perpetrators to justice, in effect a detective.  A woman has been tied to St Mungo's Cross to cure her melancholic madness, not unusual in itself, but now she is dead.  Gil's investigation rapidly becomes more convoluted, McIntosh weaves together medieval Scots with early Glasweigian life where the church ordered the hours of the day and the law of the land into an engrossing mystery with plenty of twists and turns.