Newer Radio/CD car receivers can play regular CDs, MP3 CDs (allows whole books on one CD), have USB inputs for thumb drives or smart phones (mine has 2), sometime SD cards, sometime Bluetooth streaming from a smartphone or iPad. All possible book inputs. I usually use MP3 CDs that I make and thumb drives. Besides the audio books from the library I downdownload books that are release as Podcasts a chapter at a time, I collect them till I have the whole book and
PodioBooks where you can download full books.
I also subscribe to several PodCasts that do radio plays, they are shorter but also have a place in our collection for trips.