This is one of the best fantasy novels that was written before The Lord of the Rings and The Chronicles of Narnia. It has monsters planning evil deeds, colorful settings, some unique fantastical ideas, vivid characters, and a fast paced story. Perhaps a good way to describe it is as a novel length fairy tale. A few silly ideas involving the goblins fall flat, but other than that, it holds up well. While the story is fairly simple, the author found ways to give it quite a bit of depth, and weaves in a variety of original touches, bringing the story to life and making it worth reading for any fan of classic fantasy.
There is a bit of mild violence and scariness involving the goblins. It has a few weird ideas, some of them involving mysticism. Anyone old enough to read The Chronicles of Narnia should be fine with it.
I've also reviewed the sequel, The Princess and Curdie.