The Magician's Nephew is a prequel and probably wouldn't have drawn as much interest as The Lion, Witch & the Wardrobe. The only requirement would be Tilda Swinton and Liam Neeson from the other movies.
The next few books star the same characters (each one introducing someone new), so they would have to be filmed as sequels.
The Horse and His Boy is set during the later years of The Lion, Witch & the Wardrobe, and doesn't really have a set time it neeRAB to be made in. Having said that, I don't see how or when they could make this movie, after the final movie I see them calling it a day.