You can find the options to add or delete pages from your book at the bottom of the editor.
Insert Page: To add a new pages click on the spread you wish the new pages to go before and click add. Maximum page counts will vary between book styles. Keep in mind that if you are creating an image wrap album and are still on the cover page in your editor you will not be able to add a new page.
Delete Page: Click on the spread you wish to remove and then delete. Please note that there is a minimum page requirement. Once you've reached the minimum page count you will not be able to delete any pages
Inserting or Deleting Multiple Pages: You may also add or remove pages in bulk by changing the number in the 'Total Pages' drop down menu. These will be added or removed from the end of the book.
Rearrange Pages: At the bottom of the editor you will find a view of all the spreads, click to select the spread you want to move (it will highlight in blue), then drag and drop it to the desired location. The spreads will shift over as you rearrange.