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.
This was my favorite book when I was little. I knew every word by the time I was three. No joke, I knew every, single, word, to this book! I haven't picked it up again in a while but I remember there being a lot of teachable moments for my parents to spark conversation.