Value of Appleton Universal Cyclopaedia?

We have a full set of 12 of the Appleton Universal Cyclopaedia and Atlas reference books published in 1903, a newly revised and enlarged edition. The original series has a copyright of 1874. It is in very good condition, very little use. The volumes include many original engravings. They have been in our family since the late '30s. Is there a market for these, or anyone who collects? What might their value be?