I forgot about my nano -- I have one of the little square ones (8 GB) that apple sent me when they recalled the battery on my 1st generation (4 GB) one. I've been carrying my iPhone around when I want to walk at work and it's kind of awkward. I should look for it and put some content on it.
That said, I use my 64GB iPhone for listening to audiobooks. I listen to them only in the car and I have a cable that connects between the headphone jack on the phone and an "AUX" connection on the front of my radio. I put my books -- all purchased from Audible -- on it through iTunes and then use the Audible app to listen to them.
Audible does not seem to have any good organization and iTunes buries *everything* so I can never find what I want to listen to through the phone's native music app. I organize and upload my content through iTunes and then I use Audible to listen to the books.
I have
Skullcandy earbuds that I like at lot and a work-provided
bluetooth headset that I use when I don't want to hassle with cords. However, the headset doesn't stay in/on my ear well and the sound is not as good as what I get with the earbuds.