This book is written in a modified version of Markdown, processed by Redcarpet using a system I created called Bookdown. It allows me to write text and code together, such that when the Markdown source is processed, the code examples execute. It's sort of like a reverse of literate programming. The source is hosted on GitHub.
The headers are set in Helvetica Neue, defaulting to the system sans-serif if not available. The body text is set in Georgia, falling back to the system serif font.
Code is set in Monaco or Lucida Console, depending on your operating system, with syntax highlighting provided by highlight.js.
Thanks for reading, and check out other entires in “What Problem Does it Solve?”