Long live the Greenwitch! If you believe that I am, perhaps, on a tear through a "new" series, then you're correct -- I've only recently discovered The Dark Is Rising Sequence, and I've been working my way through in my own fashion (2-1-3 thus far). Here's what the cover looked like on the book I read earlier today:
That would be Greenwitch by Susan Cooper. In which the Drew children from book one meet Will from book two, and two out of three Drew children display some sort of telepathic abilities (Barney and Jane, in case you were wondering. Poor Simon seems doomed to be kind, good, true, clever and responsible, but perhaps nothing more -- I'm sure time will tell, however.)
I had thought perhaps that the title referred to an actual witch, and perhaps it does, but if so, it's a reference to a very old kind of magic that can't be contained inside a person, which makes it spookier and wilder and cooler than the sort than can be, if that makes any sense. The Greenwitch turns out to be a sort of sacrificial offering woven by the women of Cornwall from local greenery, including rowan and hawthorne. Most of the current weavers don't understand, appreciate or believe in the old magic, yet they maintain the tradition, which is detailed in a wonderful way because Jane gets to see it come into being -- and makes a wish that is clearly portentous, although it is not at first clear whether it will turn out for the best in the end.
The children are in Cornwall (and back in Trewissick) because the object that the Drew children found in book one has been stolen from the museum, and they know why -- and are trying to prevent the Dark from winning this particular battle. An evil artist is about the town, causing trouble for the kids and for Merriman Lyon and the good Captain whose house featured so prominently in the first book.
I loved the interaction of magics in this book -- not just Light versus Dark, but Old Ones versus Wild Magic. Very, very cool. I can't wait to read The Grey King, which I have been promised by reliable sources is a masterwork.