Once Upon A Galaxy is one of the best books about the making of a Star Wars film. You can get it for about $10-20 on e-bay. Its a paperback. If you want a preview of the goodness inside, read this:

http://secrethistoryofstarwars.com/magicofempire.html

Theres a book called The Making of Return of the Jedi that was created as a sort of follow-up. Its almost as good, but Jedi just isn't interesting to learn about. Also, Once Upon the Galaxy is literally just a personal journal a guy made while on the production, so its more personal and anecdotal, whereas the ROTJ is a standard making of book, but it has access to things that the ESB book lacks, like call-sheet reproductions and private memos.