The Drowned Woods — Emily Lloyd-Jones

In this magical, ethereal fantasy novel from the bestselling author of The Bone Houses a crew led by a magic welding woman fight to take down the overpowered prince that used and abused them.​
 
Once upon a time, the kingdoms of Wales were rife with magic and conflict, and eighteen-year-old Mererid “Mer” is well-acquainted with both. She is the last living water diviner and has spent years running from the prince who bound her into his service. Under the prince's orders, she located the wells of his enemies, and he poisoned them without her knowledge, causing hundreds of deaths. After discovering what he had done, Mer went to great lengths to disappear from his reach. Then Mer's old handler returns with a proposition: use her powers to bring down the very prince that abused them both.
 
The best way to do that is to destroy the magical well that keeps the prince's lands safe. With a motley crew of allies, including a fae-cursed young man, the lady of thieves, and a corgi that may or may not be a spy, Mer may finally be able to steal precious freedom and peace for herself. After all, a person with a knife is one thing…but a person with a cause can topple kingdoms.

The Drowned Woods -- set in the same world as The Bone Houses but with a whole new, unforgettable cast of characters -- is part heist novel, part dark fairy tale.