Few people have every used TeX itself, compared to the massive amounts who have used LaTeX; but if you haven't used raw TeX you should try it sometime; it's surprisingly how simple and powerful it is.
I hate to be that guy, but... the TeXbook works and recently (finally!) had an official ebook release, illustrations and all. It’s a peculiar value of “works”—personally, I had to be stuck home sick for a couple of weeks with no reading material to get through it from beginning to end, it doesn’t really start to get rewarding unless you get through at least half of it or so, and I went away from it thinking that I like neither Knuth’s way of writing nor his approach to programming language design (“computers follow rules”). But it still has a lot of helpful stuff.
When it comes to more systematic descriptions and connections with more conventional programming-language ideas (it took me some time to recognize that TeX uses what is essentially dynamic scope throughout), I like TeX by Topic[1].
TeX itself is quite powerful, but you end up finding you're reinventing a macro package.
But you can use it basically as markdown and it formats things quite well.