My Contrary Mary - Cynthia Hand

Welcome to Renaissance France, a land of poisons and conspiracies, beauties and beasts, mice and... a queen?
Mary is the Queen of Scots and the jewel of the French court—except when she's a mouse. Yes, reader, Mary is an Edian (shapeshifter) in a kingdom where Truths rule. It's a secret that could cost her her head—or her tail.

Fortunately, Mary has a trusted ally in her fiancé Francis. But events at the gilded court take a treacherous turn after the suspicious death of the king. With Francis by her side, Mary faces a serpent's nest of conspiracies, traps, and betrayal. And if Mary's secret is exposed, heads will surely roll.