I pulled down the eight Kindle books I actually bought, about half of the books in my Kindle library are public domain, stuff like old Sherlock Holmes novels, some FAA handbooks, etc.

Next I guess is Audible. Over the years Audible has offered a lot of free trials with a complimentary audiobook several times, and I've amassed a bit of a collection. Including the edition of The Martian narrated by R.C. Bray you can't get anymore. Those I'd like in mp3 format if I can get it.