Some of my books are very old and their covers are not available in Google. How do I update cover artwork on a book?
When viewing a book, press the camera button (while in edit mode). This will give you the option to select your image source or use the camera to take a picture. It is also possible to change the artwork, by pressing the cover photo, then the camera button in the next view.
You can save images to your photo library, by searching google images from Safari. When you find the image you want, hold down the image and then select "Save Image". This will save it to your Photo Library, which you can then select as described above.