Here's my review of his book Wintersmith.
This is supposed to be a children's book. I suppose his non children's books are supposed to be for adults. However I see no reason why discerning readers of all ages wouldn't love any of this books.
This one is about a teenager, which is probably why it's classified as a children's book. As always, I am amazed by his inventiveness and treatment of deep things. In this case it is about witches and how the seasons work and what happens when the two get tangled up. It has its funny moments, but it is much more than just a funny book.