Simply Audiobooks (www.simplyaudiobooks.com) has a free trial right
now. Promo code 15FREE gives you 15 days.

Requires a CC to activate the trial, but (like any free trial) if you
cancel within the 15 days, you won't be charged anything. So, you can
get a few hundred dollars worth of audio books to listen to in your
car, and pay nothing. Enjoy!