I read basically the whole of the TeXbook (the spiral-bound book documenting the TeX language and implementation) many years ago, and found it fun and quirky, and very educational regarding how typesetting is done, and how typesetting practices can be transported into a computer implementation.

It was, for example, where I learned what an "em" is, the difference between a hyphen and an em-dash, what a kern is, what italic correction is, what leading is, and on and on.

But I found the language description not very clear. There are passages in the TeXbook that refer to different stages in processing as the stomach, mouth, gullet, etc. I later read an offhand comment that Knuth did not use a conventional lexer/parser setup in implementing TeX, and that decision made TeX more ad hoc from a language point of view. This, in turn, may have made the macro expansion setup more complex, with its expandable vs. not expandable tokens, restricted modes, etc.

I wonder if others who have more experience with language design had the same reaction?