I would like to know if there is some manual from which I can learn the bases of LaTeX programming language. I need something universal, not related to some specific package or class, that at the end allow me to have a look at various source files, to understand the code in them and even to modified some file or to create my own source files of packages or classes.
I am not a beginner user of LaTeX. I have written a lot of documents using LaTeX with some standard classes and packages and with more exotic ones. What I want is not a manual that tells me how to write a document with LaTeX but how LaTeX works at its bases.
For example what are macros and how to use them, how to use command like
\relax, the key
\begingroup, etc... (I have only open the file
article.cls and check for command that I don't know..)