Ye Old Shoppe
T-shirts and prints
Really, there is nothing old fashioned. Most of the merchandise is available at RedBubble.com.
The t-shirts with shipping run about \(30. That is not a great price. The reason is that these are created one at a time off of a high quality t-shirt printer instead of doing a run of a few hundred using screen printing technology. RedBubble overs a variety of items with the images which have a range of prices. I am taking ~\)4/shirt. Note that I have spent on the order of $60k over the years doing the research required to generate the content.
My Digital Books
From leanpub.com, a PDR, epub, or Mobi file:
Doing Physics with Quaternions, $9.99
Doing Special Relativity with Quaternions, $1.99
Amazon Books
The Theoretical Minimum: What You Need to Know to Start Doing Physics
Leonard Susskind and George Hrabovsky
Quantum Mechanics: The Theoretical Minimum
Leonard Susskind and Art Friedman
Special Relativity and Classical Field Theory: The Theoretical Minimum
Leonard Susskind and Art Friedman
Space-time Physics
Edwin F. Taylor and John A. Wheeler