From Cinescape Magazine, July 2002:
And while rumors persist that an outline for a third trilogy exists (a joke Lucas made in passing to Rolling Stone, which then printed it as a fact), the director insists that the only continuation to the saga will be in the form of licensed properties.
"There are two worlds here," explains Lucas. "There's my world, which is the movies, and there's this other world that has been created, which I say is the parallel universe-the licensing world of books, games and comic books. They don't intrude on my world, which is a select period of time, [but] they do intrude in between the movies. I don't get too involved in the parallel universe."
(Italics mine)
Online version available here.
From the introduction to "Splinter of the Mind's Eye", 1994.
"It wasn't long after I began writing Star Wars that I realized
the story was more then a single film could hold. As the saga of
Skywalkers and Jedi Knights unfolded, I began to see it as a tale
that could take at least nine films to tell-three trilogies-and I
realized, in making my way through the back story and after story,
that I was really setting out to write the middle story.
After
Star Wars was released, it became apparent that my story - however
many films it took to tell - was only one of thousands that could be
told
about the characters who inhabit its galaxy. But these were
not stories I was destined to tell. Instead they would spring from
the imagination of other writers, inspired by the glimpse of a
galaxy that Star Wars provided. Today it is an amazing, if unexpected,
legacy of Star Wars that so many gifted writers are contributing new
stories to the Saga."
(Italics still mine)
TV Guide Interview with George Lucas, week of 11/19/01:
TVGuide: "Yet novelists have written "Star Wars" sequels using the same characters and extending their stories."
George Lucas: "Oh, sure. They're done outside my little universe. "Star Wars" has had a lot of different lives that have been worked on by a lot of different people. It works without me."
(Italics mine)