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.
Le pongo un cuatro porque el trabajo de investigación de Gibson es brillante y, aquí, ha sabido resumir toda esta información en menos de 100 páginas. Lectura bastante buena para aproximarse al autor.