StarWars.com offers more peeks inside the upcoming book, The Making Of The Empire Strikes Back.
Preview 1 includes a bit of trivia that missed being published in The Making of Star Wars.
Update:
Preview 2 looks at archived marketing material, including early logo concepts for Empire.
Preview 3 spotlights the work of Ralph McQuarrie, specifically his concept sketches of "Darth Vader's castle", and his work on the cover art for Splinter of the Mind's Eye.

See also:
"The Making Of "The Empire Strikes Back"
The Making Of 'The Empire Strikes Back' - Full Cover Revealed
''Star Wars: The Making of The Empire Strikes Back'' Images