High Performance Browser Networking
hpbn.co
hpbn.co
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Well written, clear, and enlightening. This is absolutely required reading for anyone doing web development (front and back).
If you can, buy a hard copy to keep as a reference. I have referred back to mine regularly (and will surely refer back to it in the future, likely next time I have to do something serious with websockets). It's fairly neatly organized by protocol, so if you just need a quick review of HTTP2 (for example), you can skip to that section and read it in isolation.
I highly, highly recommend.
The second is "Secrets of the Javascript Ninja" by Biebault and Resig. Every one of my devs groaned at first, then thanked me profusely when they finished it.
FWIW, my favorite "look-it-up"-style resource is Speaking Javascript by Rasuchmeyer - and his follow-up "Exploring ES6".
-- this book, about NAT