SilverWook said:

I think the George Lucas of 1979/80 was a bit more unfiltered and less guarded than he is now. The whole book is an largely unvarnished look at ESB's production and where Lucas thought the saga was going. Even Carrie Fisher's drug problems are hinted at. For those reasons, I think it will remain forever out of print.

The new making of is actually even more unfiltered than Alan Arnold's superb book, I recommend Rinzler's book even if you own the old making of due to the great additional material. Rinzler got access to Alan Arnold's original tape recordings and his stories and interviews are more straightforward in some cases than what was allowed in print back then.