I read this book first when I was 11, and just reread it, considering whether it's a good book for our nine year old. For him, I will wait. For me, I was again delighted by it.
The prose is solid, it has weight without being dense.
The portrayal of light and dark, good and evil, as bigger than any normal human is a strong brew. It is presented viscerally, and without explicit moralizing.
For me, the power of the story arises for the sense of familiarity I have with the commonplace scenes of a British family at Christmas time, and their juxtaposition to the scenes of fantasy and imagination. (If I did not have that sense of affinity with the everyday life of the characters, I doubt I would be as moved by the story.)
It is not slick entertainment, it may not appeal to everyone, but it leaves me more attuned and appreciative of the good, true and beautiful in my life, and inspired to be watchful against the encroaching darkness that would distract me or obscure those things.
If you like fantasy, and have the imagination to bring story to life in your mind's eye, this book is first rate. If you just want to be entertained, and have an attention span accustom to 21st century pacing, you may find this one does not move fast enough.