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/s

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 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/generic/knuth/tex/tex.pdf https://www.tug.org/texlive/devsrc/Master/texmf-dist/doc/gen...