Written by Lloyd Alexander, they are excellent. So, so, so good. Shames just devoured all series. The High King was his favorite, followed by the Black Cauldron. We're going to feed them to Shaemus next. They're so well written, I wouldn't object to the girls reading them for school either.
Do your kids love Harry Potter? Read the Book of Three aloud to them so they're hooked (or get the audio version—it's very good), then hand them the Black Cauldron, and you've got a new series to hook them on! There's even a Black Cauldron movie as a reward (though it's pretty terrible!!!)