Complete Set of The Book of Knowledge 1935?

When cleaning out my parents' home after their deaths, I found a complete set of The Book of Knowledge (20 volumes), copyright 1935. All of the books are in great condition except the 20th volume (index) which was understandably used more than the others. It's spine needs to be repaired, but is intact on one edge. Can anyone give me an estimated value for this set?

By Susan C