I thought the books were quite good, and I really enjoyed them. Obviously preferred this series over Twilight (oh god the abuse), and I was happily recommending them to my ex-students (aged 13-16).
It is a YA novel because it introduces basic concepts to a group of people whom "wise" and "worldly" adults might find idiotic/childish. They might not have the experience and reading we have done, thus there is no basis/concept for them to fall back on.
It is kind of like how my generation grew up singing Spice Girls' "2 Become 1" with no inkling of what it means, then BAM! I learned what the song means a couple of years ago. HAH!
Anyway, a quote:
Critics who treat 'adult' as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow.
But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.
- C.S. Lewis