I first read Alan Garner's The Owl Service as a child and I remember that it scared me silly, so it was with some trepidation that I picked it up this morning to re-read it. However, I need not have worried - there's no doubt it's a peculiar story - but I didn't find it scary. Perhaps because I've read more mythological works since I originally read it aged about 9 or 10.
Garner takes a tale from the Welsh Mabinogion, that of Bloduwedd, and re-works it into a more modern setting (the book is actually set in the 1960s). Bloduwedd was a woman made from flowers by Gwydion to marry his nephew Lleu. She later betrayed her husband with a lover and for that was punished by being turned into an owl. This "love triangle" is played out relentlessly in the valley in which Garner's characters are currently holidaying. I'm not going to try to explain the ins and outs of the story as it's quite complicated, instead I'll just point you in the direction of Kimberly Bates' Greeen Man Review of the book.
If you're interested in Alan Garner, there are articles and interviews on Robert Mapson's comprehensive The Unofficial Alan Garner site. I've got The Weirdstone of Brisingamen and The Moon of Gomrath, plus The Stone Book Quartet and The Voice Than Thunders to read yet, so I shall definitely be reading more of Robert Mapson's site.