If you are patient David, excellent copies are available on ebay. I bought two copies of the 20th edition from ebay a while back which are in perfect condition.
Be aware that the two 20th editions come from different publishers somewhat like Rukl's moon atlas. If you read Ian Ridpath's editor's pages all will become clear. The links as follows :
http://www.ianridpath.com/books/nortonpage.htm
http://www.ianridpath.com/books/nortonpage2.htm
Hope you find this information useful.