I'd like to use scans/photos of old book covers in a product I'm selling. Are the laws regarding the images on the cover different from the ones regarding the content of the book? How to I check to make sure I can use the image?
Copyright is copyright. It does not matter what the content or object is. The laws apply all the same.
It is very possible that the copyright on the cover (and its components) is not held by the same person as the copyright of the contents. Both (the whole cover and contents, and individual components of both) are also likely to be licensed in some way to whoever is printing the book.
You check for the copyright by looking at the information provided in the book, and contacting whoever it lists as the rightsholder or licensee; they may or may not choose to respond in any productive manner. If that information is missing, finding the rightsholder (if one exists) is a matter of library and information science, not law.