I'm re-reading Heir to the Empire (1991) in its 2011 annotated edition, which has footnotes from author Timothy Zahn about ideas he'd come up with for the book, and whether or not they fit with later canon. One example is a mention of Tatooine being a "quarter of the way across the galaxy" from Coruscant:
A quarter of the way across the galaxy,15 Han Solo sipped at his mug and surveyed the semiorganized chaos flowing all around him. Didn't we, he quoted to himself, just leave this party?
15 As no official map of the Star Wars galaxy existed at this time (and wouldn't for many years to come), any positional relationship between Coruscant and Tatooine was pure guesswork. —TZ
(Heir to the Empire: The 20th Anniversary Edition, Chapter 2)
Since then, maps of the Star Wars galaxy have all placed Coruscant near the center and Tatooine in the Outer Rim. It seems like Tatooine's location is based on Luke's comment from A New Hope about Tatooine being the "place that's farthest from" the "bright center to the universe", but where did the idea of Coruscant being at the center come from? Was this from the West End Games sourcebooks, or was it introduced after the Thrawn trilogy?