Suppose I want to learn TeX. Then, is The TeXBook a complete source? Does it cover all relevant aspects of TeX? Or is there something worth learning about TeX which is not present there?
Does "TeX programming" also appear there?
The TeXbook is only about
It says nothing, for example, about the LaTeX or ConTeXt formats, nor about more modern engines like pdfTeX, XeTeX, LuaTeX… (Nota Bene: To be allowed to exist, all those evolutions had to be named differently than TeX and/or Plain TeX.)
It remains a very useful reading if you want to learn more about how things are made at the lower levels.