From what I have heard, Lucas wrote one big book on Star Wars. Then, he split it into 9 novels. The first 3 Star Wars movies were based on episodes 4,5,6 because they were the best to choose for the times. If pod racing were in #1 originally, it would have been extremely hard to show them back in the 70s. Also, after Vader dies, and everything blows up, etc, who says the Empire doesn't still live. Or the Sith at least. I mean, if they were to introduce Thrawn and Mara Jade in the movies, that would be so awesome.
Honestly, I can't tell you what the last 3 are, but Lucas is planning to make them, if he's healthy enough to. I'm not sure where, but I read an interview in which Lucas sai that he wanted to do all 9. So, if Indy 4 is to come out, let's hope Speilburg directs it, so Ford could still do all his stunts, and stop with all the drama movies. Harrison Ford doesn't belong in cushy cushy movies.