How much is a set of 1955 Compton's Pictured Encyclopedia Volume 3 worth?

I would be interested in finding a 1955 (or so) Compton's set for extra reading for me and my children... I read some articles from my father-in-law's 1947 set and it was very interesting. Information about the Aztecs were told in a much better way than the textbook my daughter has in her 5th grade class this year. Does someone have ideas on where I could look for something like this?