My Contrary Mary by Cynthia Hand

Welcome to Renaissance France, a place of poison and plots, of beauties and beasts, of mice and . . . queens?⠀
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Mary is the queen of Scotland and the jewel of the French court. Except when she's a mouse. Yes, reader, Mary is an Eðian (shapeshifter) in a kingdom where Verities rule. It's a secret that could cost her a head—or a tail.⠀
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Luckily, Mary has a confidant in her betrothed, Francis. But after the king meets a suspicious end, things at the gilded court take a treacherous turn. Thrust onto the throne, Mary and Francis are forced to navigate a viper's nest of conspiracies, traps, and treason. And if Mary's secret is revealed, heads are bound to roll.⠀