You can find the options to add or delete pages to your book at the top 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.
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.