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 thoroughly enjoyed this book, which I had bought for my Dad many years ago. Unlike most anthologies, the selected passages are lengthy - often several pages - rather than snippets, so it is possible to get a real sense of the style of an author. The collection is very wideranging, and is well organised.
The introductory essay is excellent in its own right.
My only slight cavil is that the end is abrupt. It needed a valedictory!