I'm currently using a portable CD player for my audio books, but find that when I'm at the GYM on treadmill, bicycle, etc., that it's too large & awkward.  I'm wanting to find something that's very basic, very simple and all I care about it doing is letting me listen to audio books.  I would like to find the least EXPENSIVE device that will accomplish this.  Does anyone have any recommendations?  Thanks.  kThere are these things called "mp3 players". I use mine for listening to audio books.
I've got a SanDisk MP3 player that runs on a AAA BATTERY (no charger needed, batteries available anywhere)  and it's not much bigger than the battery.  I use it mostly for music, but it also does audio books.

It came with an arm strap to hold it that's really HANDY when exercising or running.

There are a number of programs to convert cassette tapes to MP3 files.I have a Sandisk Sansa mp3 player. I can listen to audiobooks on it. It does though need a charger to CHARGE the battery but it is doing fine. I bought the 2gb version (which is cheapest) but you can always buy the sandisk memory card to increase space if you need it.