
I also found A Beginner's Book of TeX by Raymond Seroul and Silvio Levy helpful as a quick reference.
Also I have an anti-recommendation for Knuth's TeXBook. I did not find this book helpful at all- too much information buried in exercises (which I don't think is appropriate for a reference), and not enough directly said on how to do useful things. Figure out how to make a table of contents using only the TeXBook..
What a lot of people don't realize about LaTeX is that much of its awkwardness comes from it being a product of two very strong personalities (both Turing Award winners for unrelated work!) pulling it in two opposing directions:
• Donald Knuth, who designed TeX as a tool for typesetting primarily, allowing a meticulous author complete control over the appearance of his pages, and attempting to capture in a program (or at least making possible) the highest standards of typography developed over the centuries since the invention of printing.(†)
• Leslie Lamport, who wrote LaTeX as a macro package running in TeX, trying to hide complexity from the author and making things as convenient as possible, out of a (probably correct) belief that authors should not care about the appearance and instead only focus on the content! (https://lamport.azurewebsites.net/pubs/document-production.p...)
The result is that with LaTeX, things look “easy” superficially, but everything is implemented in TeX macros (never intended as a full-fledged programming language), so things break in mysterious ways. Not to mention the additional layers of complexity from zillions of users writing their own "packages" for everything. The error messages don't make sense to users because they often come from the bowels of TeX, and I suspect that many users even just ignore(!) warnings about overfull/underfull boxes.
Conversely, if you look at one of Knuth's own documents written in plain TeX, everything is startlingly simple: for example there is no automatic equation numbering or cross-references; for Equation 5 you just write "5" in the equation and refer to it as "(5)" later, none of that stuff with "\eqref{eq:foo}" or whatever. If macros are used, they tend to be custom for the document, rather than elaborate general-purpose ones. Consider giving plain TeX a try (a great book is A Beginner's Book of TeX by Seroul and Levy); everything makes sense, you feel in power, all the error messages are clear (often the same error messages! but now they apply to something you actually wrote/intended), and if nothing else, you'll end up with an appreciation of what LaTeX is/does.
This comment is turning into a repetition of my same tiresome comments on other sites (https://cstheory.stackexchange.com/a/40282, https://tex.stackexchange.com/a/398372, https://tex.stackexchange.com/a/386592, https://tex.stackexchange.com/a/384881, https://tex.stackexchange.com/a/518802 etc.) so I'll stop here I guess :-)
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(†): good paragraphs, avoiding widows and orphans and hyphenation on consecutive lines and loose/tight lines adjacent to each other, all of that. By far the longest chapter in The TeXbook, the manual on how to use TeX, is called “Fine Points of Mathematics Typing” and teaches the careful reader about aspects of typography that TeX does not handle automatically — and this is not even counting such advice in other chapters such as ties for avoiding line breaks in “psychologically bad” places. See for example http://www.rtznet.nl/zink/comparison.pdf (linked from http://www.rtznet.nl/zink/latex.php?lang=en) comparing plain-text paragraphs in Word and InDesign and TeX. If you look at the “bad” typesetting that Knuth rejected as painful (https://tex.stackexchange.com/a/367133), I think you get the idea :-)
Unlike LaTeX, where the (TeX) error messages usually appear arbitrary / incomprehensible / unrelated to what you're doing, in TeX (IMO) all the error messages are very informative and include a lot of information and give you ways to recover from your problem and poke around, get more context, etc. First you'll have to have read a manual (or I recommend A Beginner's Book of TeX by Seroul and Levy), but my claim is about the user experience in the steady state.
Of course, part of the reason is that LaTeX is much more complicated than the low-level things one may be doing with plain TeX. Another reason is that the LaTeX authors were working with severe constraints, one of which was of their own choosing: their choice of using TeX macros as a “programming language” (which it was never intended to be, and at which is it horrible). Nevertheless, a big part of the reason is that they were trying so hard to make things "easy" for the user in the typical case that they didn't care as much about ways in which things can go wrong and how surprising errors can be.
• It is indeed true that the first edition of Volumes 1–3 of The Art of Computer Programming and the second edition of Volume 1 (1968, 1969, 1973, 1973) had been done with hot-metal typesetting (pieces of lead, laid out to form the page) with a human doing the typesetting/compositing (with a Monotype machine). And that for the second edition of Volume 2 in 1977, the publisher Addison-Wesley was switching to a cheaper alternative. But the cheaper alternative was not electronic; it was phototypesetting (the letterforms made with light and lenses instead of metal). It was Knuth who noticed a book that had been electronically typeset (Patrick Winston's AI book), realized that electronic typesetting machines actually had enough resolution to print like “real books” (unlike typewriters or printers of the time), and decided to solve his problem for himself (as he felt that he could make shapes with 0s and 1s).
• The summer of 1977, Knuth spent in China, thinking he'd specified his program in enough detail and his two students would be able to complete the program by the time he came back. He came back and saw they had only implemented a small subset, and decided to spend his sabbatical year writing the program himself. He mentions that as he started writing it he realized the students' proto-TeX was an impressive effort, because there was a lot missing from his specifications. Knuth's article The Errors of TeX (reprinted in his collection Literate Programming) goes into excellent detail on the development process.
Also: a suggestion to try plain TeX (instead of LaTeX), if you haven't tried it. The book A Beginner's Book of TeX by Seroul and Levy is especially good. It might surprise you.