I think I really leveled up when I started writing more libraries as well as trying to understand more about how things work. e.g. really understanding prototype inheritance. I'm a armchair compiler nut, so I would also refer to the ECMAScript spec quite often too. One thing that I've found helpful in teaching others is actually reviewing what ES6 -> ES5 compiled code looks like from Babel and understanding the choices it makes, particularly when in spec-compliance mode where the output can be a bit verbose.
I'm not a big book person, so I can't recommend any on JavaScript unfortunately. I have heard lots and lots of praise for the "You Don't Know JavaScript" series, but I have not myself read them: https://github.com/getify/You-Dont-Know-JS
I've found egghead.io to be pretty awesome, and they have lots of free content too. I recommend them so much I sometimes wish they had some sort of referral program lol.