Court of Dragons by Frost Kay

As a daughter of the Dragon Court, Wren knows three truths: never show weakness, pay the tithe, and never trust the elves.
When the enemy strikes on the eve of her wedding, Wren has no choice but to fight. Captured, she’s dragged before the elvish king and given two choices.
Death or marriage.
She submits, biding her time as she plots and spies, determined to strike at the heart of the elvish royalty and take back her family’s throne once more. But the more time she spends with her dangerous and achingly handsome husband, she finds herself wavering.
Only the strong survive the hash moors of the Dragon Isles, and she can’t afford to question her loyalties, leaving only one final choice.
Strike hard and fast. Never waver. It’s his heart or hers.