We’ve got several books on retro programming and hardware design. They’re available on:
Amazon (print and Kindle)
Electronic books on Amazon are in Print Replica format, which may not be compatible with digital ink e-readers.
You may be able to send a PDF from GumRoad to your e-reader. Download a sample PDF and email it to your Send-to-Kindle address.
A reflowable ePub version is available from alternative (non-Amazon) ebook vendors.