The Highwaymen, Gary Monroe 2001 (I own this)
Harold Newton, by his sister, Rosetta Humphries 2004
The Buckner Brothers, by Ellis Jr, 2004
The Highwaymen, Bob Beatty, 2006 softcover 2006
The Highwaymen, Beatty, expanded Hardcover edition, 2006
Harold Newton, Monroe 2007
Al Black's Prison Murals, Monroe 2009
The Highwaymen, Catherine Enns, 2009
You could say there are actually nine if you count the Ft Lauderdale Museum's publication of Highwaymen, Newton and Hair which kicked off their exhibit in 2006.
We won't count the Backus books, three of those plus a high quality exhibition catalog. He's not a highwayman, despite what some folks tell me at shows.