There have been tons of books written (I'd say hundreds but many are from other time periods in the star wars universe so it's hard to say) about the future of the galaxy after ROTJ ends. If you're really interested in "What happens next" then I suppose you could read the Jedi Academy trilogy or the New Jedi Order series. I used to read the young jedi knights when I was in fifth and sixth grade *reminisce-sigh* Anyway the series does go on after the trilogy, but I don't think there will be any more movies.