The Witch in the Well. Number 10 in series

When Catherine LeVendeur's grandfather sends for his family to tell them their well is going dry, Catherine is alarmed. The family's wealth depends on its status, and if the well goes dry, their castle will fall. Her grandfather seems wracked with a fear deeper than that, though - something connected to the mysterious woman who appears at odd moments, who may or may not be the family's ancestral ghost.
Catherine doesn't believe in the magical legends her family has handed down over the years: that they are the descendants of a Charlemagne knight and a faerie. She puts her faith and distrust in the human condition. And when bodies begin appearing - not ghostly spectres, but the freshly dead kind - Catherine knows she is right. She must uncover the secrets of the witch in the well .