By "unauthorized" I mean "not by C. S. Lewis."
For example, Neil Gaiman wrote a (lovely in my opinion) short story titled "The Problem of Susan," collected in his fragile things short story and poems anthology that gives an alternative interpretation of the Narnia stories.
Can you suggest a definitive list of commercially published sequels, full length parodies (a la Bored of the Rings videlicet The Lord of the Rings), or explicit reinterpretations of (e.g. Wicked: The Life and Times of the Wicked Witch of the West, and The Wonderful Wizard of Oz) Narnia or C. S. Lewis' visitors there as of June 1st, 2015? (No need to cite another story beginning with a girl and a wardrobe culminating in discovering and adventuring in another world. :)