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.
There is a lot of interesting information, but author keeps on getting back to already mentioned stories or spontaneously telling about different stuff in the middle of a different story. That makes the book quite hard to read (well not hard, it's just annoying).