js folder within
Know the fundamentals
Angular supports the MVC structure and is really easy to get started with. However, WordPress coders, at times, are not very familiar with the MVC structure and as such, React seems to be an easier and more apt option. Furthermore, since React uses just the View component, it is popular among designers as well.
Decide what you need
For WordPress developers, the needs can basically be split as backend or frontend. If you are dealing with frontend development, frameworks such as AngularJS, React, or Backbone.js will suffice for your needs. You can find a longer list with more options here.
For backend coders, however, especially ones who are already well-versed with PHP, Node.js is a skill worth acquiring.
Where to learn
For learning React, I have found React for Beginners to be the most well-planned and structured resource. Superhero.js is another amazing collection of resources and tutorials that you can learn from.
In terms of books, the options are plenty, and you can find some of the most useful and popular ones on this GitHub thread.