You can purchase a physical copy on Amazon, which is sold at-cost (in accordance with the license), you can read the web version, or you can download the pdf or epub from the Releases page.
The book itself is built using bookdown, which is part of RStudio.
You can build the pdf via make pdf, the ebook via make epub and the web version via make website.
To modify the cover.pdf, you'll need to install the Google Font Squada One. Download it, extract it, and move SquadaOne-Regular.ttf to ~/.local/share/fonts/.