Additional pages may be added in the Book Editor. Hover over an existing page in the Book Editor, then click on either + icon on the sides of the page. A new page will appear either before or after your selected page, next to the + icon just clicked.
Dragging one or more images from the photo collection pane towards the end of your page thumbnails will cause a new page to appear. Drop the image on the new page to generate a layout containing the selected image(s).
Pages may also be added in the Page Editor by clicking on the + icons on either side of the page. The new page will appear on screen.
You can also generate new pages by using the photo collection's Timeline mode. Timeline allows you to divide your images (organized chronologically) into per spread layouts - generating a new pages each time a split is made.
If you add too many pages to your album, you'll receive a warning when you review and finalize your project. You can set the maximum number of pages for your book in the Settings tab for your custom sized projects.