A childrens' story published in 1871 By George MacDonald.
Highly recommended for adults as well, this is the sort of thing I wish I had had the chance to read when I was a kid.
MacDonald's writing style is fluid and lyrical-- you'll be able to read it right quickly without a particular struggle.
When I first read it, I thought "Why hasn't this gotten a Newberry Award? A bit of research reveals that it was published in the wrong century, on the wrong continent.
This in no way diminishes the quality of the work.
It has rather a bit of thinly-veiled allegory in it, so if C. S. Lewis turns you off for that reason (I've heard some people say he does) this might not be for you.
I personally found the thoughts rather enlightening, and a thoroughly reasonable treatment of religion and tolerance.
The sequel to this book is The Princess and Curdie, another fine piece of work.
Lewis fans will note that these books are mentioned (practically by name) in his novel, That Hideous Strength.