To create a cover, all you have to do is: 

1. Create a new book
2. Tap the cover and upload a photo/write a title by tapping "Image" and "Book title":
   
   
   
3. When you are happy with your cover, tap "Save cover":
   
4. Done! 

You can change the color on your cover and shuffle the layout once you have added a photo or a title.