The Dark is Rising

Susan Cooper's The Dark is Rising series is comprised of five books: Over Sea, Under Stone, The Dark is Rising, Greenwitch, The Grey King, and Silver on the Tree. It is a masterful job, in my opinion, of integrating Arthurian mythos with otherwise unrelated Celtic myths (including large swaths of the Mabinogion). It's well worth your time, and is far better-written, I think, than the vastly more popular (to the point that it's ludicrous I'm even making this comparison) Harry Potter series.