Though there are some that actually overlap
RotJ, like Mara Jade: By The Emperor's Hand
(it was a comic series, not a novel).
Lucas originally planned a nine-film series, then cut it down to three films (the OT). Later, when technology reached a certain point, he redid the OT to make it closer to what he initially wanted (the SEs), and decided to do the prequel trilogy.
Hopefully, he'll never decide to do the followup trilogy, for two reasons:
1. Lucas is very proprietary about his toys. If he goes into the post-RotJ era, he would
contradict everything he could find, throwing what might have been the greatest Shared-Universe in the history of Storytelling out the window (he doesn't realize what he has
in the Lucasfilm-enforced continuity
2. The Star Wars Saga (OT/SEs and prequels) works
better as six movies than it would as nine. With the prequels, the entire series is now about Anakin/Vader (not Luke & co). Anything after RotJ would just seem tacked-on.