Well Witched by Frances HARDINGE

Take your British folktale about a wishing well as background peeking into a story set in modern times. Add in three adolescents who have worked out "the pecking order" and their individual roles in their group just to have imposed gifts from the witch in the well destroy the balance and threaten their world. The kids steal coins from the well and consequently are set the task of granting the wishes which go with the coins by the witch. Thoughtful musings among the characters about wishes and what people really want when they are wishing. Scary fantasy.