I've actually contributed to the Ray Tracing In one Weekend project. The process is pretty manual. The book is actually written HTML, and the code snippets are duplicated from the source (not linked in). Before release, it's a manual process to go through each book and read/build it from scratch.

In my opinion, the open source nature really is the key thing here. Most of the issues are small things (typos, confusing verbage, etc). So it get's refined as people read it.