@LukeAlmighty @LukeAlmighty There are a total of 15 books. The movie is roughly based on the first 6 (IIRC). The books were written something like 40 years before the move.
Actually, L. Frank Baum was tired of the series after the 6th book and wanted to end it and thus permanently closed travel between our world and Oz after Dorothy left the emerald city. However, the public demanded more and bills were due so he invented ways for other characters to go to the Oz despite travel being cut off between worlds permanently.
In the books that the movie covers, Dorthy actually travels to Oz and back home 6 times. The movie just turned it into one long adventure with multiple events they encountered along their journey to the emerald city which in the books Dorthy doesn't even go to the emerald city until the 6th book on her 6th visit to Oz.