Narnian fanfiction - some suggestions
A child who wrote to C.S. Lewis, asking him for more Narnia stories, was encouraged to make up some of his own. Many writers have taken Lewis at his word, writing for their own entertainment - trying to fill in some of the most obvious gaps in his narrative, or trying to fill in the back story of some of the minor characters.
This kind of writing, where fans of an author write their own private stories based on the author's published works, is usually called fanfiction.
One of the largest fanfiction sites with a Narnian section, is fanfiction.net.
(Choose Books and then choose Chronicles of Narnia. The latest submissions will appear in chronological order.)
Some of the writers just try to fill in what could have been additional stories in the author's own "universe". Others like to experiment with "what if"s - what if Edmund was an adopted brother of the three other Pevensies, or what if Caspian and Susan had fallen in love (much encouraged after the Prince Caspian movie came out, I'm afraid). I usually prefer those who stick to Lewis' own universe - but even some such "alternative universe" (AU) stories can be well written and worth reading ...
Graphics by "Beginte" at NarniaWeb
One of my favorite writers on fanfiction.net writes under the penname elecktrum. She was elected "best all-around author 2008" in an election at Narnia FanFiction Revolution. (One of her stories won the "best new story 2008" award in the same election.)
Awarded by Narnia FanFiction Revolution
Background graphics is courtesy of Eos Development.