Yep, it's written in Scribble [http://docs.racket-lang.org/scribble/]. Which is magical for writing substantial documents. You notice the build time for your book has crept up to 30 seconds because you've forgotten to run the separate compiler in a while. You do it — which compiles all the chapters (taking a little over 30 seconds) — after which subsequent compiles are back down to 4 seconds. Good luck getting that with most other writing tools.