Challenge of the Unknown (Perry Rhodan #32) By Clark Darlton Books / Paperback Publisher: Ace Books, January 1973 Price $5.53 Free Shipping Send to a friend Collectible - Good New York: Ace Books, 1973. Book 32. 16mo mass market paperback. Good book. Inquire if you need further information. $5.53 1 in Stock at Warehouse Quantity Add Free Shipping to continental U.S. OR $1.00 per item discount if shipped to store.