Vintage Material
Here you'll find some materials (esp. links) in regard to the earliest history of US66 and it's predecessors.
1913 Arizona National Old Trails (this is a link to Jeff's Bygonebyways website)
National Old Trails (some material about the National Old Trails Road)
1916 National Old Trails (vintage ACSC maps)
1926 National Old Trails Road (vintage ACSC maps & guide)