Kingdom of Dance by Deborah Grace White

Don’t get caught eavesdropping on a dragon—or dancing might be the least of your problems.

With eleven younger sisters, Princess Zinnia usually has little choice but to be responsible. But if she had her way, she’d spend her time exploring the seashore and cavorting with the dragons often found there. When she sees one of those dragons flying mysteriously over her castle one night, the temptation to sneak out after it is too strong to resist. Except, by the time she realizes she doesn’t recognize this dragon, she’s already witnessed something much bigger than she was ready for. And now she’s trapped, her sisters along with her.
Lieutenant Obsidian hates using his magic. But when his king specifically seeks his help to uncover the reason behind his sister’s uncharacteristic deception, Obsidian has no choice but to answer the call. To his surprise, the deeper he delves into Princess Zinnia’s secrets, the more he finds himself wanting to help her. And his magic might be the only way to do that.
But someone else wants Obsidian’s magic. Someone determined to see Entolia fall. Someone with much more power than either magic or crowns can fight. And if Zinnia and Obsidian can’t find a way around the magic that binds them, and soon, more kingdoms than theirs will pay a terrible price.