Greenwitch is the third book in The Dark is Rising series by Susan Cooper. In Greenwitch the main characters from the first book, Over Sea, Under Stone, and from the second book, The Dark Is Rising are united. They meet on vacation in Trewissick arranged by Merriman Lyon. At first the Drew children, Simon, Jane and Barney, resent the presence of Will Stanton. They think he will interfere with their quest to recover the stolen grail. But as events unfold, they realize he has an important role to play in defeating the Dark too.
The next book in the series is The Grey King.