Destined for the Fae King by Megan Van Dyke

The fae king needs a bride, and Mira is determined to be his choice.

Mira Rivera has dreamed of running away to Faery since she first discovered her ability to see the fae as a child. But family duty has kept her firmly among humans. However, the king of the Court of Fire has requested eligible young women journey to Faery and compete to become his bride. It’s the chance of a lifetime, and Mira isn’t about to let it pass her by.
But her dreams are nearly crushed before the contest even begins when Lysandir, the king’s mysterious and handsome younger brother, attempts to deny her a place in the competition. The prince confounds her at every turn and is a distraction she doesn’t need, especially once he changes his tune and tries to make amends. He should be the very last person she’d ever want, but the more time they spend together, the more Mira realizes it’s the quiet and thoughtful prince stealing her heart, not the enigmatic king she’s competing to wed.
When rising tensions with the Unseelie dark fae turn deadly and trouble brews between rival courts, a treasonous affair could damage the Court of Fire and destroy any chance Mira has of making her family proud.
But turning her back on true love feels impossible, even if it means defying a king, her family, and fate itself.

This slow burn forbidden romance set in Faery can be read as a standalone.
However, this book discusses events from books 1 and 2 in the series and may spoil elements of those books if this one is read first.