Most materials about LaTeX I tend to find on the web seem to be of the 'programming by StackOverflow' variety, i.e. 'copy this snippet of code, tweak it to your needs and compile'; they don't teach you about what the code actually does, how it interacts with everything else, and there is no syntax specification apart from series of digressions like 'oh, by the way, if you wish to achieve effect X, you can use option…
(Links to the other currently mentioned documentation: http://mirrors.ctan.org/graphics/pgf/base/doc/pgfmanual.pdf (of which TikZ is a sub-/co-package) and http://mirrors.ctan.org/macros/latex/contrib/memoir/memman.p... ; and 2ion already linked to the KOMAScript documentation (https://news.ycombinator.com/item?id=12830476).)