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.
We-ell.... early Victorian 'erotica' -for want of a better term, in a bound edition of an underground victorian magazine. The Victorians were not so strait-laced as you would think!