In Death's Shadow — Siobhan Muir

After three hundred years, Princess Maia is done with dryads.
Imprisoned by the wood folk for a crime she didn't commit, Princess Maiasora Silvercloak wants her life back. If she finds the Key to the Twelve Realms and returns it to the dryads by the summer solstice, she's free. Too bad she has no clue where to look. And the dryads' solution might kill her.
Master Assassin Quinn Tarlen wants safe passage across the dryads' forest after his last contract put him in the crossfire. The dryads offer him a deal: Escort a princess on her quest to find a magical artifact and he'll be released. One catch: she's the daughter of the king who killed his brother.
Spell-bound together, they must scour the world for the pieces of this artifact without getting killed or killing each other. Falling in love isn't part of the plan as they race against time to secure the Key before the deadline. But nothing's ever simple when magic is involved and death isn't the worst outcome if they fail to find the Key.