Sure! The entire source code of TeX has been published as a book called TeX: The Program, which is Volume B of Knuth's Computers and Typesetting series (the TeX and METAFONT manuals and source code, and the definitions of the Computer Modern fonts): https://cs.stanford.edu/~knuth/abcde.html#abcde The major part of the book (just the program itself, without the book's 1-page preface, 2-page bibliography, 6-page instructions on how to read the program, mini-indices on each two-page spread, 1-page diagram of approximate memory requirements, and hand-tweaked typesetting) are also available in any TeX distribution or online e.g. http://texdoc.net/texmf-dist/doc/generic/knuth/tex/tex.pdf

It is written using a system called WEB, and Knuth calls the idea literate programming — see article at http://www.literateprogramming.com/knuthweb.pdf and the WEB manual at http://texdoc.net/texmf-dist/doc/generic/knuth/web/webman.pd...

All this is somewhat hard to read for most modern programmers IMO. In my free time I hope to make these more readable, and I've started with simply reading and annotating the simplest of the programs written in WEB: https://shreevatsa.github.io/tex/program/pooltype/ https://shreevatsa.github.io/tex/program/tangle/ — if anyone reading this is interested let me know (create an issue on the GitHub repo or something).