Addendum: In England Everyone Has a Motorcycle

I actually got this before the trip, but forgot about it.
Here's one of the articles mentioned by Shepherd in the book, from the archives of the LA Times. I tried to find the article from the Kansas City Star as well, but their archives aren't online, and my visit there didn't allow for a trip to the newspaper.

There also a 1919 article from Motorcycling and Bicycling Magazine about him available from Google Books, a 1922 mention in the journal Engineering and Contracting, and a handful of other things. He seems to have been a member, while he was writing Across America, of the Birmingham Natural History and Philosophical Society.
Shepherd's name is also attached to the out-of-print book "Full Story of Capt. Hinkler's Death and Return," and the L.A. Times article mentions another, previous book, "Through England By Motorbike," which I would love to get my hands on.