I should also consider that maybe there are public domain copies available. Let me check the original copyright status. In the US, works published before 1927 are in the public domain. The original 1950 publication would have a copyright term of author's life + 70 years. If the author is dead, and the copyright has expired, then it's public domain. Ian N. Sneddon passed away in 2001. So if the copyright was owned by the author or the publisher, the term is author's life + 70 years, which would be 2001 + 70 = 2071. Therefore, copyright is still in effect. Therefore, even the 1950 edition is still under copyright.