If found “TeX by Topic” a great introduction, it's less overwhelming than the TeX book. Also really helpful in wrapping your head around macro expansion is the `texdef` cli tool, which allows you to quickly evaluate a line of macro code. Finally, https://www.tug.org/utilities/plain/cseq.html is nice for reference.

There are three books that are worth looking at to get plain TeX. I’d start with Viktor Eijkhout’s TeX by Topic (which I think is free in PDF on CTAN). Then the TeXbook to really get everything (the PDFs that float around the net are illegal. Please buy your copy. DEK donated his royalties to TUG so your purchase will help support the continuation of things like CTAN and the LaTeX project). And finally, if you really want to dig deep, Stephan v. Bechtolsheim’s four-volume magnum opus, TeX in Practice will tell you more than you ever wanted to know.

I can only hope to curse you with a TeX nerd-snipe :) I can recommend reading the first few chapters of The TeX Book (it's a genuinely fun read) followed by the links in the article's footnotes for more operational information.

I found The Advanced TeXbook by David Salomon to be helpful. It shows how to make your own format on top of plain TeX, including table of contents, page numbering, cross-references and insertions.

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..