There is no automatic function to alphabetize your content, however it's easy to rearrange the pages within your book by going to your "Organize" tab.
Once there, click on the chapter/section you'd like to organize, and the pages will appear. From there, just click on the page you want to move and drag and drop it into it's new spot.
Please note: If a page is called "New Page" within the Organize tab, it means it still has the default name it was given when you added it. It can be renamed at the top of the page (if it has a title) or to the right of the page under "Page name."