If you've ever added custom album artwork to an iTunes song, you get the basic idea of how to change the cover art of a Project Gutenberg book in your iTunes library. If you haven't, I'll quickly list the steps here:
- In your iTunes library, select the Books category in the source list.
- Select any book that shows it has no cover, right click on it, and select "Get Info."
- Select the "Artwork" tab.
- Drag and drop any image into the white box on the artwork tab and click Okay. Voila! You've just added a custom cover to your ebook.