To understand the basics of SFML it's important to work through the relevant official tutorials. They should be addressing the parts from different perspectives and equip you to use the SFML classes in any use case:
https://www.sfml-dev.org/tutorials/latest/For books I personally recommend SFML Game Development with which you'll create a basic 2D game and learn about different patterns on how to structure a game.
The master branch is still work in progress, so I currently wouldn't recommend to learn game development or even SFML with it right now, as it's subject to change and you might run into issues, which you can't even judge if they are related to your code or some issue with SFML.
Either stick with SFML 2.5.1 or use the relatively stable SFML 2.6