"Greenwitch" is the third book in the Dark is Rising sequence by author Susan Cooper. It follows the adventures of a young boy named Will Stanton and his friends as they seek to recover a powerful artifact known as the Greenwitch. This artifact is tied to ancient magic and is crucial in the ongoing battle between the forces of Light and Dark. The story is set in the coastal village of Cornwall, where the characters encounter various mythical elements and face challenges that test their courage and friendship. The narrative explores themes of sacrifice, loyalty, and the connection between nature and magic.