I don’t think anyone has mentioned the Earthsea trilogy by Ursula LeGuin yet. (She has added a couple of books to the trilogy since, also).
Oh, I don’t know about that concerning Will. I think Susan Cooper wrote as much growth into him as possible; after all, he was an Old One, and a lot of his powers and wisdom–so to speak–were inherent. That was part of the character setup: he was both young and old at the same time–which explained some of the mistakes he made, and the mature manner in which he handled them.
If you look at The Grey King (which won a Newberry), you’ll see Bran’s character transformation, as well as John Rowlands’ changes through that book and Silver on the Tree. While the books did focus on Will, sure, (and on the Drew children in Over Sea, Under Stone), I think Ms. Cooper’s greatest accomplishment for the series was portraying other characters’ reactions and changes after dealing with Will and his order of Light.
I loved them, at any rate. 
For the on-going list, I suggest Baum’s Oz series, Doris Buchanan Smith’s A Taste of Blackberries, most of Judy Blume and Beverly Cleary’s books, Nancy Garden’s Prisoner of Vampires (and definitely Annie on My Mind if the child is old enough), Ellen Raskin’s The Westing Game, and The Witch of Blackbird Pond, by Elizabeth George Speare.
Hi - I’ve realized I’m a children’s/YA bookaholic too, even tho I’m - yikes - 50. The best are much less pretentious than “adult” literature. While there are some mentioned here that I hated (Pullman’s pretentious trilogy; C.S. Lewis I just never got into), there are also many that I cherish: Skellig, Wrinkle in Time.
My additions to the favorites list (just off the top of the head, mind you) - The Devil’s Arithmetic, Tuck Everlasting, The Giver.
Has anybody read Jenny Nimmo’s Snow Spider trilogy? It’s another of those Welsh-mythology-based stories, somewhat reminiscent of the Dark is Rising sequence (and with one character who I’m thinking must be based on the same guy as the fellow in The Black Cauldron who didn’t like anyone but his horse). The writing is pretty good.