Justin de Quincy, book 1: The Queen's Man
Sharon Kay Penman1996

Epiphany, 1193. Eleanor of Aquitaine sits upon England's throne. Her beloved son Richard Lionheart is missing, presumed dead--and the court whispers that her younger son John is plotting to seize the crown.
Meanwhile, on the snowy highroad from Winchester, a destitute young man falls heir to a blood-stained letter, pressed into his hand by a dying man. The missive becomes Justin de Quincy's passport into the queen's confidence--and into the heart of danger, as he pursues a cunning murderer and jousts with secret traitors in Eleanor's court of intrigue and mystery. . .