The day of the serpent / Cassandra Clark.

Enhanced descriptions from Syndetics:

The murder of a loyal king's man threatens the self-crowned King Henry's new regime in this second gripping medieval mystery featuring friar, sleuth and reluctant spy Brother Chandler.

January, 1400. The bowman strikes at night, slaying one of King Henry's loyal garrison men before melting back into the darkness. Was the murder the result of a personal quarrel? Or is it, as Henry's stepbrother, Swynford, fears, the start of an uprising against England's self-crowned king? Swynford orders Brother Chandler to investigate, before the spark of rebellion can set the whole country alight.

Friar, reluctant sleuth, and even more reluctant spy, Brother Chandler is a man with dark secrets and divided loyalties. To the murdered King Richard. To his paymaster, the usurper King Henry. And to beautiful, naïve Mattie, a maid in the household of heretical poet Geoffrey Chaucer, who holds dangerous secrets of her own.

Trusted by no one, Chandler must walk a tightrope of secrets and lies if he is to uncover the truth about the murder, while ensuring he - and the few people he cares about - stay alive.

Combining rich historical detail with deep characterisations and enthralling mystery, this medieval puzzler is a perfect choice for fans of sleuthing monks and nuns like Ellis Peters' Brother Cadfael and Peter Tremayne's Sister Fidelma.