Having set up a Moodle book you are presented with a new page: Add new chapter.
On this page, add a Chapter title. (Since this is the start of the book you could type 'Introduction' here.)
Then add text to the Content field. The editor includes icons for various formatting functions. You can click on the Reveal or hide additional toolbar items icon (three dots at the end of the first row) to reveal a second row of functions.

When you've finished click on Save changes.
This generates a Table of contents at the top of the left-hand side of the screen. The title of the first chapter will appear here and the content will appear in the main area of the screen.

To add another chapter, click on the + (add or plus) symbol alongside the chapter you've just created in the Table of contents.
This will take you back to the Add a chapter page.
Again, fill in the Chapter title and Content fields, and click on Save changes.

As you add more chapters you can:
You will also see forward and back arrows at the top of each chapter to move through your book.
We've focused here on adding text to your course content. Now you're going to look at adding links to the text.