The WordPress Codex is the central repository of all the information that the official WordPress team has published to help people work with WordPress.
The Codex has some basic tutorials for getting started with WordPress, such as a detailed step-by-step discussion of the installation, and lists of every template tag and hook. Throughout this book, I'll be providing links to specific pages within the Codex, which will deliver more or advanced information on the topics in this book.
The Codex can be found at https://codex.wordpress.org/Main_Page. Refer to the following screenshot:
Apart from the Codex, there's also one more resource that will come in handy to new and experienced developers alike. It's called the WordPress Code Reference, and it can be found at https://developer.wordpress.org/reference/. It delivers a lot of docs on WordPress' functions, classes, methods, and hooks.