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 enjoy every book in this series though I do not agree with many of the subjects mentioned. It's a bit outdated but I think it's a great set to help you find topics to farther explain and read more of.