I’m sure you’ve all read The Dark is Rising series by Susan Cooper (and everyone but me seems to have already known she wrote more stuff than that, too; how did I miss that fact for so long?) (Still haven’t gotten around to reading anything else by her, but seriously, any time now.)
Anyway, the second book of the series, also called The Dark is Rising, is still probably my all-time top pick for Second Book That Is Better Than The First Book.
If it’s been a while since you read this series, Maureen’s got a brief review of the whole set up over at By Singing Light.
I agree, though the series was a high point in my reading as a kid, the ending did leave a lot to be desired. And if you want to re-write the ending in your head, Maureen links to a fanfic story that can do that for you.
Also, I think this series contributed to my feeling that really, no one needs to bring King Arthur into their original fantasy. I almost always hate that — to me it feels like the author is just trying to throw in All The Cool Stuff whether it fits or not, and the actual original fantasy world gets cluttered.