The Wicked One by Nadine Millard

Widowed Philip Everwood, Earl of Breton is hoping that escaping to the quiet countryside will help his son recover from a recent tragedy. Something is plaguing the boy, and Philip is desperate to save them both from the nightmares of the past.
A chance encounter with the gypsy woman who lives in the woods may change things for them both. They call her The Wicked One. But for Philip and his child, she might just be salvation.