A remarkable number of excellent books about Front End Web Development are freely available online, if you know where to look. Here's a collection of those that I've discovered, and deigned to keep in my bookmarks.

Eloquent JavaScript:

http://eloquentjavascript.net/ eloquent-javascript

Learning JavaScript Design Patterns:

https://addyosmani.com/resources/essentialjsdesignpatterns/book/ ljsdp

High Performance Web Sites:

http://stevesouders.com/hpws/ hpws

High Performance Browser Networking:

https://hpbn.co/ hpbn

Designing for Performance:

http://designingforperformance.com/ dfp