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 have the 3rd edition teachers volume and all the workbooks and overheads. This is the text I taught my 1st year spanish students from. I still have my lesson plan and all the tests and quizes.