My personal love affair with The Dark is Rising sequence, by Susan Cooper, started when I heard the radio adaptation. It's certainly an adaptation: characters and plots missed out and truncated, details changed to suit the format. But the voices of the characters in my head are still the ones given to them in this adaptation, for the most part. Especially Merriman. If anyone wants to listen to it, send me a PM and I shall arrange it if possible.
I didn't read the books until I was sixteen or so. Oh, I think I came across them, maybe even read one of the five, but I didn't sit down and read them in order until then. Over Sea, Under Stone was okay: an Arthurian Enid Blyton story, almost. The real love affair started with Will Stanton's story, The Dark is Rising. I was fascinated by the myth and magic and history that Cooper wove together. I was fascinated by Will himself, both an ancient creature and a young boy, all at once. I loved his quest, and I loved his family. I loved the feeling I got, even at sixteen, that I should probably crawl into the safety of my bed.
Greenwitch draws together characters from both of the preceding books, and here again was realism -- antagonism between the young characters, jealous belligerence -- and magic, all at once, and here was Will, on another quest.
The Grey King is probably my favourite, though. The Arthurian elements of the series kick up another notch, and Will finds himself manoeuvred into Wales. The morality isn't black and white: the most truly good character is not one of the Light, the supposed good, but a human being. The Light is revealed as cold and cruel, in a way, despite the way it does everything for the best. And my favourite character joins the series, Bran Davies, with his pride and arrogance and pain and -- I admit this is a big attraction to him for me -- his Welshness. There's a level of this story which the child reader will not necessarily understand or relate to, of pain and difficulty, in the figure of Bran's father, Owen Davies.
Silver on the Tree brings everything together -- all the characters, all the magic, all the little hints from the other books. Each of the characters has a part to play in the fight, even the human ones. And again, there's very adult pain and betrayal. One of my favourite quotes comes from this final book:
"For Drake is no longer in his hammock, children, nor is Arthur somewhere sleeping, and you may not lie idly expecting the second coming of anybody now, because the world is yours and it is up to you."
The fandom is small to non-existent. There's fic, but not a steady supply of new stuff. I write fairly actively for it, but for so many people, it's a series they read as children, with young characters, that they wouldn't think of making their actual fandom. But there's so much here to play with, so many lovely character dynamics -- some which are more explored than others in canon, and others which could be explored further. There are so many amazing characters, even the minor ones -- Stephen Stanton! John Rowlands! -- that deserve attention. There's so many heartbreaking angles to what might happen to Will after the events of the final book. There's so much growing up the characters still have to do.
(Personally, I'm probably an oddball. The main pairing I've seen in fic is Will/Bran, which I do adore, but I also like Bran/Barney and Simon/Will and Will/Jane and Bran/Jane and...)
A fic rec, to coax people in: The Ascent, by Annakovsky. Gen, plotty, a bit of a fix-it to a part of the end that people often don't like.
You can find my more detailed reviews of each book here: Over Sea, Under Stone, The Dark is Rising, Greenwitch, The Grey King, Silver on the Tree.