Nice to see Alan Garner's Wierdstone of Brisingamen mentioned.
+1
The Owl Service is still of my favourite books, ever – sparely written & very dark. I was lucky enough to see him give a talk about Thursbitch a few years ago. He's a national treasure.
The BBC series was very dark too, and has recently come out on DVD, I believe. I think Elidor was made into a TV series as well. There's a sequence in either Wierdstone or Gomrath, can't remember which, where the children have to pass through the mines of Fundindelve, which never fails to induce an extreme sense of claustrophobia in me. Brilliant writer, and, as you say, national treasure.