Books can be attributed to "Unknown" when the author or editor (as applicable) is not known and cannot be discovered. If at all possible, list at least one actual author or editor for a book instead of using "Unknown".
Books whose authorship is purposefully withheld should be attributed instead to Anonymous.
I bought this in Cuba and found it a very nice vacation memento. All of the tourist hotspots there sell books from this publisher. I wish I had bought more.