Via Starwars.com:

SEE AND LEARN THE STORIES BEHIND MORE LITTLE-KNOWN ART FROM A STAR WARS MASTER.
Most Star Wars fans are aware that Ralph McQuarrie painted the cover for the first printing of the movie tie-in issued in the fall of 1976, six months before the film’s release. And that he also painted the cover for the first Expanded Universe novel, Splinter of the Mind’s Eye. But few know that he developed an unused cover for the novelization to The Empire Strikes Back, that one of his paintings was used on an early mock-up for the Return of the Jedi novelization, or that he did preliminary design work for the first of the Lando Calrissian adventures. This three-part series explores both the published and unpublished Ralph McQuarrie Star Wars artwork for Del Rey books from 1976-1983.