The Spellcoats  - Diana Wynne Jones
I read this recently -- I always hesitate to revisit favorite authors from my youth, but this didn't let me down. To be sure, the plot is both elementary in fantasy terms and standard for Jones (it seems like almost every single one of her early books involved the main character discovering the hidden magical talents they were born with), but it's really well told. The four main characters (Tanaqui and her siblings Hern, Duck, and Robin) all have distinct personalities, they interact with each other just like they'd been together all their lives, they're realistically children, of the ages they're said to be, and grow during the course of the story. The pace of the plot never flags, and it's based on some interesting mythology, with a final note linking it to the later history of Dalemark that broadens the fantasy world.