There are a lot of answers that are using a somehow cryptic syntax that I am not familiar with. Is there any Not So Short Introduction to TeX (like Not So Short Introduction to LaTeX) ?!
My book "TeX by Topic", which is strictly about TeX, not LaTeX, is available
- as a free download, including sources
- from lulu.com, nicely printed
Definitive reference for the locations of all this:
WORK IN PROGRESS: texprogramming-persian is the ultimate reference for anyone interested in TeX programming, written in Persian. I have finished writing it on paper and I just have to typeset it which may take between 2--3 months considering my limited time resources. It is licensed under GPL version 3 and I have used several references for it including TeX by Topic and TeXBook.