While reading this question and its answers I tried to find where, for instance,
\@ifnextchar is documented. I'm afraid I couldn't... I looked in TeX: The Program and TeX for the Impatient which can be found in
texdoc(link) and of course in Google, but found no real documentation. My question is where is it documented?
Actually a broader question is where similar internal commands of
(La)TeX are documented? I encounter in TeX.SE many answers which involve core components of
La(Tex) but I usually fail to find a rigorous documentation of these elements.
This makes it more difficult to have deeper understanding of the answer, and in particular try to modify/adopt the solutions to slightly different problems. So the main question is: Where core command are documented?
Edit: Let me fine tune my question since most of the answer aimed mainly at the
\@ifnextchr. What is a (the?) canonical documentation of both
LaTeX, where magic like
\@ifnextchr and similar
@ involving elements are documented? Is it only TeXbook? What is the
LaTeX equivalent of TeXbook?