Come up with a back cover (optional, but nice)
A list of things that could go on the back cover
1. Photo/Bio of BBEG. The ultimate challenge your players may never get to
2. A page of the most used tables, diagrams, stats, etc. a one page DM screen
3. Make it a continuation of the front cover
4. For a small upcharge make the rear cover customizable with the purchaser's PC
5. Your bio. Tell them why you're the best game designer and what else they should buy from you.
7. Repeat the Front Cover.
8. Mirror image the Front Cover.
9. The Back Cover is the "back" of the Front Cover
Picture the physical manuscript being glass and looking through it from the back, you see the "back of the front cover as if it was painted on a glass store window. That image is your back cover.