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.
A feast for the eyes. However, as the introduction acknowledges, about 80% of the book covers 1500 to 1850 (Renaissance, Baroque, Landscape, Classical, Romantic). It is also almost exclusively concerned with the gardens of royalty and nobility.