I started coaching adults at the Exeter club in 1993, about the same time as Alan Maynard started up the current incarnation of Exeter Junior Chess Club. I went looking for some useful resources for teaching, and there were some, but mostly I became a magpie, picking shiny bits out of various good books. I did find it irksome that so many books repeated familiar examples, and I thought I could at least pull those out for my colleagues, and that became the core of the Canon. I found particularly useful:
* Tony Gillam - Simple Chess Tactics and Simple Checkmates
* Euwe/Kramer - The Middlegame I & II
* Kotov - Think like a Grandmaster
* Nunn/Griffiths - Secrets of Grandmaster Play
* Livshits -- Test your Chess IQ 1 2 & 3
* Littlewood - Chess Coaching

My frustrations with the available literature were:
* dreadfully partisan openings guides -- "Winning with the Petroff" is a title I won't forget in a hurry
* lots of material written for juniors which was far too advanced in content and language
* lots of material written about openings by people who have no experience with the system in question ("I must give this a try myself one day" was one memorable comment by a GM author)
* lots of material written to instruct with it seemed no idea about how to teach apart from show and tell -- no tests, no summaries... (Pioneering efforts by Bellin failed to carry the day)
* if there were books of tests, they were just tactics
* and frankly, a lot of crap -- database dumps, over-familiar examples, superficial content, and if you were lucky, all of those in one book.

Now, we have an abundance of riches, and a lot going free. We have books, videos, CDs, DVDs, databases, playing engines, tactics rating sites, online arenas, streaming... too much to choose from. Reviewers cannot cope with the avalanche of material! There are also several laudable cases of a GM retiring from play, going into coaching, deciding all the available material was garbage, and developing some excellent resources for themselves -- Aagaard's stuff being the prime example. But I am pleased to see that some of my gripes from my early days have been addressed:
* there is lots more thoughtfulness about the business of instruction -- books have introductions, summaries, callbacks, review quizzes... I thought Aagaard's book on the Panov was a fine example.
* intelligent use of computers -- as sparring partners, revision guides, and database tools. Chess Position Trainer was very helpful for this.
* there are now some excellent active learning materials -- Chess Choice Challenge was great (if pricey for what you got), the recent Steps materials in particular are very good and thorough, and I like also Gambit's Workbooks for Kids (still too hard for most juniors, though) -- and about each dimension of the game
* and there is material now discussing how to teach and learn about chess -- Hawkins, NiC, Rowson, James, Hendricks and Waizkin have all made commendable (if contradictory) contributions in the last 10 years.

I expect you could teach yourself chess these days just from free YouTube videos (although again the chaff/wheat ratio can seem a little high: "Basic Openings for Black: Sicilian Dragon & King's Indian: Simple and Sound" seems an offence under the Trades Descriptions Act). I don't like video so much as a learning medium myself -- I'm an old man, and still prefer gall ink on parchment -- but I am very happy to hear youngsters learning about and discussing new openings they have seen online, without them having to lash out a lot of pocket money on a possibly unsuitable book to do so.