The Lymond Chronicles, book 2: Queen's Play
Dorothy Dunnett1999

Second in the legendary Lymond Chronicles, Queen's Play follows Frances Crawford of Lymond who has been abruptly called into the service of Mary Queen of Scots. Though she is only a little girl, the Queen is already the object of malicious intrigues that extend from her native country to the court of France. It is to France that Lymond must travel, exercising his sword hand and his agile wit while also undertaking the most unlikely of masquerades, all to make sure that his charge's royal person stays intact.