I know that there are a already number of questions on this site about good instructional (La)TeX materials for users at different levels.
What I am interested in, though, is what set of documents will give me (in which reading order) as complete an understanding as possible of TeX and LaTeX. If one would like to know all, why bother with "intros" anyway ;-)
I was gonna write that for TeX the best first document (and, in the sense described above, technically the only necessary one) should be "The TeXbook", but then there's e-TeX, which is not covered. And I don't know to what extent there is other, hidden advanced knowledge one can only learn only from various advanced tutorials or from looking at the source code.
Let's, for simplicity, assume that I am excluding knowledge about LaTeX packages from this question. For those, the best way is to read the package documentation. One should skim the package documentation of certain frequently used packages (those one is using) anyways.
Once one has an understanding of (e-)TeX, I have the same question for LaTeX 2e.
One reason for asking this question is that I never cease to be amazed at what gems of obscure tricks there exist. There's gotta be a way of acquiring this knowledge in a most efficient way other than by "reading around randomly and perusing the source code".