when listening to an audio book, how do I back up just a few seconds on the nano?

On my 5th gen nano (which is not touch screen), you can do that by pressing and holding the Back (previous) button. If your nano is 6th gen (with touch screen), I would try touching and holding the equivalent button on the screen. On my iPhone running the iPod app, you can "back up" within a song by touching and holding the Back button on the screen.

If you tap the now playing screen so that you see the books artwork displayed, tap on that screen once and then if you swipe left, you will see the progress bar at the top and below that, an icon with a circle and a 30. Tap that and it will go back 30 seconds or drag the progress bar to any point that you wish to go back to.