The Queen's Man

Description for THE QUEEN'S MAN by Sharon Kay Penman:

Epiphany, 1193. Eleanor of Aquitaine sits upon England's throne. Her beloved son Richard Lionheart is missing, presumed dead--and the court is whispering 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 bloodstained 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... .

This is #1 in A Medieval Mystery.