This book is produced using a hacked up version of Gitbook (hacked so that the styling is more in line with build.webmaker.org).
To edit the content, just edit or create new markdown files in the github repo. On git push to the master (default) branch, Travis will run a job and push the built HTML files to an S3 bucket that is pointed at by the book.webmaker.org domain. Details of that job are in the .travis.yml.
For through-the-web usage, you can either use the Github editor, or prose.io
This version of Gitbook only supports markdown files, but I believe you can have HTML embedded in markdown files.
Feedback welcome.