I always loved Over Sea, Under Stone, the first in the series. The Dark is Rising bugged the crap out of me because I hate "hero has to grow up by going through a carefully-worked-out set of adventures" stories. It felt so manipulative to me. And then I don't really remember the others.

Have you read Philip Pullman's His Dark Materials trilogy? It has its flaws, but in some ways it's a more mature and sophisticated version of the Cooper series.