React.js revolves around the concept of components, which are reusable and self-contained building blocks of a user interface. The book delves into creating both stateless and stateful components, exploring how data flows and triggers changes in your application's UI. You'll learn to build complex interfaces by composing smaller, manageable components, making your codebase more organized and maintainable.

React.js introduces JSX, a syntax extension that allows you to write HTML-like code within your JavaScript. The book explains how JSX simplifies UI development and improves code readability. Furthermore, you'll dive into React's reactivity model, understanding how to use lifecycle methods to control component behavior and efficiently update the DOM when data changes.

React.js Essentials goes beyond the basics by introducing the Flux architecture, a popular pattern for managing data flow in React applications. You'll learn how to utilize stores for managing application state and how components interact with these stores to retrieve and update data. Additionally, the book covers testing your React components using the Jest framework, ensuring the reliability and stability of your applications.