Metal by Example was published as an e-book on Gumroad in October 2015.
As of September 2016, you can name your own price. As of March 2023, after selling almost four thousand copies, I made the decision to remove Metal by Example from sale. You can download the PDF here for free. More than seven years after publication, it is quite out-of-date, but I will keep it here indefinitely for posterity.
The sample code for the book is available as MIT-licensed open source and is hosted on Github.