WebGL only provides 3D rendering capability. It is simple, straightforward, and insanely fast. It is good at what it does, that is, rendering 2D and 3D graphics. It is a low-level programming interface with a very small set of commands.
WebGL is not a game engine like Unity, Cocos2D, or Jade. A game engine has many other features, such as collision detection, ray casting, particle effects, and physics simulation. Using WebGL, you can create your own game engine.
WebGL provides functionalities to draw basic primitives such as lines, circles, and triangles that can be used to draw any complex 3D object. It does not provide a direct function to add a camera to your scene. However, a camera class can be evolved to do the same. This is what this book will help you with. It will help you create a library on top of WebGL tailored for creating games and gaming functions.