If you click through the links to purchase, you'll be able to see that purchasing his books are through third-party distributors which are all secure.
The payment forms were in HTTPS from the beginning, but the webpages (hosted at GitHub Pages). Now it's all HTTPS with a proper domain certificate, thanks to CloudFlare. Enjoy!
Chrome still complains that the payment form is using non secure elements on the page but anyway I bought the book.