I never fully read this one but it's really good, probably required reading if you're serious about webdev
CSS: BEM/OOCSS/SMACSS methodologies, they make you think critically about the structure of you styles. Harry Roberts does good writeups on maintainable css.
I second Don't Make Me Think and I also liked Eloquent Javascript (that zoo has a bunch of new exotic animals since, so I don't know if it's the best bet. It's a fun book though)
Before diving into React or Phoenix, take a step back and survey the state of the Web APIs. From hardware and device access, to client side storage, web of things, gpu compute, and high performance peer nets. There are infinite possibilities to combine different technologies into new innovations. And new additions arriving almost daily to become inspired by, such as SharedArrayBuffers and Atomics.
My advice is to find a topic you genuinely enjoy that is much in demand, perhaps web payments or security, and really gain some mastery in that area. Blog about your progress, publish articles and tutorials and of course build things! Then you will be on your way to becoming an authority and the go-to person for advice.
Two good blogs to get started are Mozilla Hacks and HTML5Rocks:
If you're a backend developer who gets frustrated by the whack-a-mole quality of doing frontend layout with your current understanding, I recommend http://book.mixu.net/css/
If you're starting out with PHP then read http://www.phptherightway.com. In fact I'd say read it even if you're not using PHP as most of the concepts still apply.
Modern has many conflicting frameworks of code organization, so learn the basic mouse trap: html,css,js and then take the initiative to learn a ListProcessor (Lisp) based language and unclog your brain by mastering map, reduce, and apply.
The tools are still primitive in my opinion, but precision is how computers behave. Making the leap toward Functional Programming has changed my coding life. Before recommending any more books, I would recommend watching Rich Hickeys talks on InfoQ about the general state of programming in general with such classics as "Are we there yet"
Essential reading might as well be direct links to helpful source code. Imitating the masters is not a poor approach, but studying their way of practice and doing so yourself is far superior.
To learn, I'd start by absorbing the https://www.w3schools.com web site. Also looking at the rendering of pages in google web tools or similar will let you know what you're getting into. For server side look at any modern rest framework and get started with that e.g. Django rest, .net web api, rails etc.
Eloquent Javascript is outdated; it does not cover ES6. The book is well written and it has nice problem sets but a reader will end up frustrated by the obsolete information.
Object Oriented Javascript by Antani and Stephanov is more current
High Performance Websites and Even Faster Websites http://shop.oreilly.com/product/9780596529307.do http://shop.oreilly.com/product/9780596522315.do
These two on website performance are good, though they're a bit dated (pre http2 etc).
High Performance Browser Networking http://shop.oreilly.com/product/0636920028048.do
I never fully read this one but it's really good, probably required reading if you're serious about webdev
CSS: BEM/OOCSS/SMACSS methodologies, they make you think critically about the structure of you styles. Harry Roberts does good writeups on maintainable css.
I second Don't Make Me Think and I also liked Eloquent Javascript (that zoo has a bunch of new exotic animals since, so I don't know if it's the best bet. It's a fun book though)