[BGthaOG]

There is also the "You Don't Know JS" series, which is free online. I have heard positive things about it, the last installment includes ES6.

From the linked README:

"This is a series of books diving deep into the core mechanisms of the JavaScript language. The first edition of the series is now complete.

[...]

These books are being released here as drafts, free to read, but are also being edited, produced, and published through O'Reilly."

https://github.com/getify/You-Dont-Know-JS/blob/master/READM...

[IneffablePigeon]

I'm a big fan of this one for getting to grips with the nitty gritty basics of JavaScript - in particular it's my go to when I forget how prototype inheritance and `this` work exactly for the umpteenth time.