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.
My mother gave me this book. Why? Do I like cleaning? No, I try to do it as little as possible but still have a decent home. Anyway, it's full of eco-friendly cleaning suggestions and ideas.