Yeah this what I was going to ask because I remember years ago being given a graphic novel by my cousin which said that the Empire wasn't defeated like we saw at the end of Episode 6 with everyone partying that was just the beginning of the end. Don't know how true that is.
There's the "expanded universe" which is basically all the official stuff that isn't in the films (books, graphic novels, games etc.).
Short story is that after Episode 6, there's a split - the Rebels set up a "New Republic", but some planets stay with the Empire (or what's left of it).
To be honest, I haven't really kept up with any of the expansion stuff for about 12 years - by which time the books covered 20+ years after the last film, and it's kept going since.
So there's a lot of material for new films out there. However, I think they'd probably be better off just cherry-picking bits and pieces rather than trying to follow anything too closely - a bit like the Marvel films have done.