In 2006, David Anthony started the first of a planned 3-part sequal to the orignal 1939 Wizard Of Oz movie.
In Search Of Dorothy takes place about twenty years after Dorothy went back to Kansas, and Scarecrow is now Emporer of Oz with the help of Tin Woodman, and the Lion as the Captain of the Guard. Scarecrow longs to see Dorothy again, and develops a Tornado Travel Machine to try to find her in Kansas. At the same time, an old woman comes across the Wicked Witch of the West's crystal ball abadonned in the woods, and becomes possessed by her spirit. The old woman catches a ride with the Scarecrow as he activates his tornado gizmo to go to Kansas. However, he ends up in a different part of the country. Scarecrow is lucky though because its conveniently Halloween time, and everyone he meets thinks he's a guy in a costume. He befriends a bull named Bull and a tree named Tree(real original names!), and manages to find out that Dorothy is at a conference in Kansas trying to convince a bunch of scientist bigwigs that Oz was a real place. This is more than a little weird, because you'd think if a farmgirl went around saying that she went to some imaginary fairyland, that she'd get institutionalized. I guess the people in this story were a little more optimistic about how people would treat someone that most would see as being delusional. Meanwhile, Lion leads an expedition to see why there is a shroud of darkness surrounding the Wicked Witch of the West's old castle, and discovers that its guarded by a fearsome beast where the Witch's spirit is raising her forces from through her crystal ball. Scarecrow manages to find Dorothy, although her niece Audrey is kidnapped by the old woman under the Witch's power. The old woman makes off with Dorothy's ruby slippers back to Oz, so Scarecrow, Bull, and Tree go after her.
This leads into the sequal, The Witch's Revenge, where we're introduced to the Good Witch of the South(Glinda's the North one in this). The third part of the trilogy is currently in production. I think this trilogy would appeal to fans of the 1939 movie, although Oz-purists might be a little mixed about it. This first book might seem better when read with the second, but I'd still recommend it on its own too.