When in Project View, you can tap Book Options and select Rearrange or Delete Pages.
Alternatively you can perform a long tap to bring up the arrange mode, indicated by the “X” buttons appearing on the corners of the pages.
Then, a simple long press on any page lets you drag it to wherever you’d like.
To delete, just tap the X icon and confirm that you’d like to delete it.
To add additional pages select Add Two-Page Spread in the Book Options menu. New pages will not include any images, so you’ll have to add them. You can only add a two-page spread at a time to ensure the book’s format remains consistent.