I don't know of any way to do it for the Kindle app.
If you got the books through the iBooks app, it is possible. Most of the Gutenberg books are available in the iBooks store. After the books have synced back to iTunes on the computer, you can right click on them, choose Get Info, and change the book cover in the Artworks tab. Or you can drag it to the album cover art at the bottom left of the main iTunes screen. A second sync will add the cover in iBooks.
I do this all the time for iBooks titles. I use a Google Image search for the book, and then drag the image I want to iTunes.
Thanks for the advice above - I've been using the same steps on my iPod for a while to get cover art on the album labels, so should have figure that the process would be similar.
I did download a couple 'free' books from the Apps store, changed the cover art, and did the syncing - work fine! Next time, I'll check the App store first.![]()