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.
Incredibly interesting and in-depth introduction to the history of Concorde for those who (like me) knew pretty much nothing about this supersonic aircraft, other than that "it made a lot of noise". Also (surprisingly) not at all dry for a non-fiction book about aeroplanes(!) Lewis has somehow managed to write this biography like a novel, painting the scenes of history's events as though you were standing there in time. Would highly recommend to all – especially those who aren't aviation aficionados.