[patrickg]

I suggest the source code of TeX. Not new, but still very interesting to read.

source that needs some postprocessing (tangle/weave):

http://mirrors.ctan.org/systems/knuth/dist/tex/tex.web

PDF from the source (including hyperlinks)

https://www.tug.org/texlive/devsrc/Master/texmf-dist/doc/gen...