I personally like the ESB book the least. It doesn't delve into the rough drafts enough for my liking, and details about some of the creative decisions taken during the writing of the film seem obscure, perhaps deliberately so. (I doubt Lucas would want Rinzler revealing that Vader wasn't meant to be Luke's dad all along.) However, the excerpted material from the story conferences is well worth reading. The ANH and ROTJ books are both great.