"Once Upon a Galaxy" has probably the best behind the scenes info on Empire. It was written journal style by the publicist on the set, literally from the beginning of production and into post production. Whether it ever gets reprinted is anyone's guess. If the recorded audio interviews still survive, they would make for great Blu Ray extras someday!
I've toyed with the idea of scanning the entire book, as I fear it would be edited to reflect George's current take on things.