I remain fascinated by the box set that simply left out Over Sea, Under Stone. It sort of makes sense, because the other books are more like The Dark Is Rising than Over Sea, Under Stone, so it sort of gives you a better idea what you're getting into... but also it must be very strange when Simon and Jane and Barney pop up in Greenwitch, and it seems clear that you're supposed to remember them from somewhere, except that book wasn't in the set so you don't.