Pedantic note, which I'm writing just in case anyone misunderstands your recommendation:
"The TeXbook" is the user manual for TeX. It is not the same thing as the book containing the (literate-program) source code for TeX, which is called "TeX: the program".
(Both are excellent, but in quite different ways, and someone looking for interesting code to read would likely be disappointed by the TeXbook, which doesn't contain that.)
"The TeXbook" is the user manual for TeX. It is not the same thing as the book containing the (literate-program) source code for TeX, which is called "TeX: the program".
(Both are excellent, but in quite different ways, and someone looking for interesting code to read would likely be disappointed by the TeXbook, which doesn't contain that.)