Wow, so many memories in this thread - Weirdstone, Dark Is Rising, Narnia, Box of Delights, The Night Kitchen, Wizard of Earthsea, The Borribles (but not The Borrowers - could never got on with them for some reason) and so on and so on.
Not seen E Nesbit mentioned, used to love her stuff, also, The Neverending Story was pretty scary as a child - the nothing that was eating everything, a difficult concept to get your head round back then I think. The Lewis Carrol stuff was also magic and headfucking and I think someone mentioned Milo and The Phantom Tollbooth. I used to LOVE reading when I was little.
Also, does anyone remember something about a kid who had nine lives and there was kind of a magic school and stuff (bit like Harry Potter I guess) but he was the best because he was unique and had nine lives in this one dimension instead of one each in nine different dimensions like most people. There was an older magician called Chrestomanci who was the only other person with nine lives and he was the most powerful guy in the world. The young kid with nine lives had them stored as matches in a special book but he refused to believe it was true so he lit one of the matches and he caught fire - that bit stuck with me.
Also, was there something called The Silver Sword trilogy or similar, guess it was sub-Arthurian legend but I loved it at the time.