Here’s a bit of news that might be of interest to some: I’ve just updated the staging repository to be able to build the weave program and use it to extract the XeTeX reference manual from the processed WEB source code — the XeTeX analogue of TeX: The Program. I needed to patch up some typesetting mistakes to get it to actually compile, which leads me to believe that people don’t go to the trouble of actually creating the book very often.
I’m not checking in the generated file, so in order to actually generate the TeX file you need to have a checkout of Norbert Preining’s 30 GB mirror of the TeXLive Subversion repo. But if you have that, you can build the reference file and then run:
tectonic --format=plain state/sbuild/merged.tex
to create a 756-page PDF of densely documented source code. Handy when trying to decipher what the C code is up to!