The primary image should almost always be the front cover of the book. If there is a dust cover, then the front of that counts as the Primary Image.
The second image should be the spine of the book in vertical format as it would sit on a bookshelf. If there is a dust cover, then the "spine" of that counts as the 2nd image.
The third image should be the back of the book. If there is a dust cover, then the back of that counts as the 3rd image.
If the first 3 images were of the dust cover, then it may be removed and the same process may be repeated for the book without the dust cover.
This should be of the Title Page. The Title Page is usually near the front of the book (but rarely the first page) and often contains the Title, the Subtitle (if there is one), the Author, the Illustrator, and the Imprint or Publisher. Note: If the Title Page spreads across two pages, then it is suitable to upload them both as one image. (See this image for reference)
This should be of the Colophon, or, the Copyright Page. This is usually on the verso (the rear) of the Title Page.
These images should be of the Contents, or Chapter titles, and any other pages that help identify the book.