Neither the 5th generation iPod nor the iPod Nano will work with my suggestions. Neither of them can use the iBooks, Music, or Audible apps.
To get audiobooks on those devices you will have to download them from the iTunes Store in iTunes on a computer, then sync them to the devices. It's been so long since I've done this that I don't know what the exact sync settings would be, but with the devices plugged to the computer and selected in iTunes, you should be able to explore the various settings and figure it out.
There should be a audiobook category, album, or something on the iPod where the book will show up. Like I said, a really long time since I've done any of this.
Note: It is possible to side load (import) audiobooks into iTunes from other sources, provided they are DRM free. In that case it's important to right click on the file, choose Get Info, and go through the tabs to make sure the file is identified as an audiobook. Otherwise it will probably act like a music file and lose all of the audiobook playback features.