I'd recommend reading the source of things you're working with (or want to work with) first.

If you're doing front-end work, then React and Redux (or Angular, or jQuery, or whatever.)

If you're doing Node.js, try reading through the lib/ folder of Node's source (the js code goes in lib/, the c++ goes in src/), and follow that with packages like express and request.

When you hit an interesting/confusing/etc. dependency, track it down and read it's source too.