I recently watched the movie, and I had both Princess and Curdie books from childhood (long gone now). They were republished with a blurb from CS Lewis on the cover. Good books, still hold up today.
And I remember Red Moon, Black Mountain sooo much! It is one of the grimmer children's books -- maybe it was for adults, but it was in my school library. I should re-read that: books are so much more understandable when you get older. "Penny! Pen! Where's Nick then?" or something like that.
There was another book with that same grim feel to it, about some English kids who visit some ruins, and the youngest girl is somehow affected by the ruins, so that mentally she is a primitive and doesn't recognize her family. That's only in the first chapter. Does anyone reading this know that book?
And Alan Garner's The Owl Service -- dark Welsh mythology-based fiction. Not enough for adults, perhaps, but scary for an 11-yr-old.