The kids and I just finished listening to Over Sea, Under Stone, by Susan Cooper. This is the first book in the Dark Is Rising sequence. We spend a lot of time in the car getting to activities, so this is a great way to listen to some books we might not otherwise read.
Amazon lists the book for grades 5-7, and while Thomas (8) wasn't nearly as interested in the story as Emily (11) was, I don't think there's anything in the story itself that would preclude younger readers from enjoying it. It does have a slow beginning, in my opinion. If I'd been reading it aloud, I probably would have stopped. I feel a book needs to grab you from the start, especially when you have two children with different maturity levels and interests to keep entertained!
The basic storyline is of three children finding themselves in the middle of a mystery and a struggle between good and evil, while on vacation at a beach house in England. Somewhat reminiscent of The Lion, the Witch and the Wardrobe, but still enjoyable. There is a lot of tie-in to Arthurian legend in the novel, as well.
The three children's activities are key to accomplishing the goals of the forces of good, for now--there are several other books to look forward to. The rest of the series titles are: The Dark is Rising, Greenwitch, The Grey King, and Silver on the Tree. There is also a movie called The Seeker, made in 2007, based on The Dark Is Rising.