We sometimes get asked if Book Creator is limited to just 3 pages. That would be awful, wouldn't it?!
When you first create a new book, you start on the cover. You'll see an arrow button on the right which you can click to get to page 2, then page 3. At that point, you'll see a + icon which you can press to add new pages.
You can also add new pages by clicking the Pages button in the toolbar. Then use the 3 dot menu to copy, move, insert or delete the page. Click on the Back button to go back to your book (or if you're using the iPad app, tap on a page to go back to your book).