In addition to the wonderfully enlightening text which I've referenced above, the concept art in the Making of ROTJ is also quite good and mostly never before seen.

One standout is the designs for Jabba the Hutt. While Phil Tippett's rather cartoonish design (from a maquette) was ultimately selected, Ralph McQuarrie surely gets the nod for the most terrifying Jabba concepts. One of his pieces features a wormlike Jabba with a maw full of slimy tentacles reminiscent of Cthulhu; another has a more humanoid Jabba, but with immense revolting folds of naked, sagging flesh, protruding eyes set farther apart than in a human face, and a mouth full of razor-sharp fangs.

There's also a piece of Slave Leia concept art by Nilo Rodis-Jamero that I've never seen before: the precursor of the final costume design, but with the top specified as made out of leather instead of metal, and Leia going barefoot. (Still better than one of Rodis-Jamero's earlier ideas for the costume, featured on the Blu-rays, which included a transparent mesh fabric top, drawn complete with visible nipples.... a wardrobe later given in modified form to Oola after apparently being deemed too risqué for the leading lady.)

McQuarrie's art for the Imperial Guards is also very interesting; he favored robing them in black and making their helmet shape resemble Vader's. It would seem Rodis-Jamero in his final design opted for red because, having been raised as a Catholic, he found it amusing to put the Emperor's minions in outfits resembling liturgical vestments. The Royal Guards outfitted in solid red, for instance, call to mind the scarlet robes of cardinals.