For one, everything was changed from Markdown to Asciidoc and the entire book can now be generated in multiple formats in the asciidoctor toolchain. Also, we're using O'Reilly's Atlas platform to generate amazingly high-quality PDF, ePub and Mobi versions automatically with every push to master. In every language. This is a massive improvement over the previous version.
The other really interesting thing is the production process. We used GitHub and prose diffs for the entire production of the book. I think we used about 100 Pull Requests to get to where we are now. This is a massive improvement over how I collaborated with editors for the first version, the first few chapters of which were actually done by sending Word documents back and forth.