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.
Read this for a project I am working on (reading books from the 90s of every century). Surprisingly interesting considering I had zero interest in this subject going in.