If adding and replacing images is easy, adding and editing text is easier! You can do everything you need directly within the browser window.
For example, take the big bold headline of each section. To add your own text into those headlines, simply click into those boxes. You'll want to highlight the default text and delete that first before you begin typing your own copy.
Alternatively, if you've written your copy elsewhere—say, in a Microsoft Word document or a plaintext file—you can just copy that text and paste it into any text space on your book page.