Natural gas is the primary source of hydrogen now. Methanol might do, either directly or as converted to hydrogen. There are cars already that can run on alcohol or gasoline at the flip of a switch. That isn't hydrogen, but alcohol could run the fleet, and does, if I hear right, in Brazil.
Just found a book you may find interesting...It's not on cars but flight. WINGS OF THEIR DREAMS (Purdue In Flight) by John Norberg Published by Purdue University Press...
Love the dedication:
"To the Purdue people of yesterday and today who have dedicated their lives to flight and to the next generation of visionaries who will fashion tomorrow on the wings of their dreams...."
Now it's time for Hoo mama to do a little dreaming...Night