I can't even begin to explain how much Diana Wynne Jones' books meant to me as a child.
My science-fiction-reading librarian aunt gave me The Magicians of Caprona and Power of Three the Christmas I was ten, and then continued, patiently and benevolently, to find me Jones' other books, including a copy of Charmed Life that must have been (in the 80s in America) like hunting down the Grail. Or the Snark. I don't know how many times I read Witch Week, which may be the only school story I found as a child and adolescent in which the main characters were the loners and oddballs, instead of the popular kids. I love Jones forever for Theresa Mullett and Simon Silverson--for representing the popular kids the way I experienced them, instead of the way they were presented in Enid Blyton (and would later be presented in the Harry Potter books).
And it wasn't just Witch Week. All of her books centered on kids who were unpopular, weird, lonely. I didn't like Power of Three as much as The Magicians of Caprona when I was ten, but I later came to love it dearly because it was so deeply about kids who were outsiders.
And Jones never collapsed into pathos; the counter-example is Menolly in Anne McCaffrey's Harper Hall books (which I also loved as a kid); everyone picks on Menolly except for the handful of characters who we know are special because they protect her. But Jones' outsiders are like Charles Morgan, who is just as selfish and self-protectively hostile as a kid in his situation would naturally be, and her "ordinary" kids, protagonists like Vivian Smith in A Tale of Time City or Caspar in The Ogre Downstairs or Jamie in The Homeward Bounders, all have to learn what it's like to be outsiders, Vivian by being thrown into a completely foreign culture, with only oddball outsiders Jonathan and Sam as her guides; Caspar by switching bodies with his loathed stepbrother Malcolm; and poor Jamie by becoming a permanent outcast. And in books like Power of Three and Charmed Life, we also see that the "ordinary" kids are human beings, too: Gair's popular siblings, Ceri and Ayna, are fiercely, lovingly partisan to Gair, and Janet turns out to be the best thing that ever happened to Cat. (Witch Week is probably the most Manichean of Jones' books in this regard, and even there, Estelle changes sides to become Nan's friend, and of course, at the end, popular and unpopular kids alike turn out to be witches.)
The other thing I came to appreciate more and more about Jones' work as an adolescent is that her books are frequently, quietly, about very adult subjects. Power of Three, after all, starts off with the murder of one child by another, and Charmed Life is, very quietly, mostly subtextually, about abuse. Gwendolyn is abusing Cat, even if Cat himself never quite recognizes it. And Fire and Hemlock, which may be, for me, the most impressive of Jones' books, is all about stupid things adults do in sexual relationships, and the way those things affect children. Not to mention The Time of the Ghost, the most autobiographical of Jones' novels, which is openly about a horrendously dysfunctional family and the terrible things the daughters of that family do in attempting to survive.
Jones taught me a lot about unreliable viewpoint characters. Cat in Charmed Life is the strongest example. There's something wrong with Cat, as Janet worriedly notes, but we never see it head on because Cat himself can't see it. Sirius/Leo in Dogsbody is another example, although I can't reread Dogsbody because it makes me cry. But I remember the way Jones shows the reader things that the viewpoint character can't understand, and it's brilliant.
But mostly, I just loved her books, whole-heartedly, as a child, and I love them now. I love Chrestomanci and his dressing gowns, I love Howl and Sophie, I love Vivian saving the world almost despite herself. I love Howard in Archer's Goon and his relationships with his siblings, epitomized and concentrated in his little sister Awful (and Awful is awful, and I love her for it). I love Polly in Fire and Hemlock learning that being a hero is about not letting humiliation stop you. Jones taught me so much--about being a writer, sure, but more importantly, about being a human being, about being kind and compassionate and about doing the right thing, even when you don't want to.
Thank you, Diana Wynne Jones. Thank you from the bottom of my heart.