Diana Wynne Jones (1934-2011)
Her first book, Changeover (o.p.), published in 1970, was actually aimed at adults. Her first juvenile fantasy novel was Witch's Business (o.p., 1973), which was aimed at younger children, around ages 8-12.
Probably Jones' most famous book right now is Howl's Moving Castle (1986, republished in 2008 by Greenwillow), which was made into an animated film by Hayao Miyazaki in 2004.
My favorite of Jones' books is Fire and Hemlock (1985, reprinted in April 2012 with an introduction by Garth Nix), loosely based on the legend of Tam Lin.
Polly, the protagonist, has two sets of memories. In the primary set she is a recently engaged Oxford student. While packing for her move to the college, she comes across a book and a photograph which apparently were once significant to her. Suddenly, she's overcome by a wave of memories of things which she's certain didn't happen.
At the age of nineteen, when the memories return, she realizes that the story was reality, with the members of her family and the members of his musical quartet each playing a role. Very cleverly, Jones constructs the book in four parts, which also reflects the quatrain construction of a ballad.
It's a very complex story, and the plot is very difficult to summarize. But to read it is to become part of Polly's two worlds, as unsure as she what's real and what isn't, who's good and who isn't. Beware, there is some danger that you won't put it down until you've reached the end.