Thanks for all the suggestions. I expect we will be able to come up with something that will work, though probably nothing that will pause playback automatically if an incoming call is received. I think I'm leaning towards e-books from the library systems and Overdrive for now, as that is most similar what my wife has been doing, and doesn't add any expense or a lot of preparation time. I briefly looked at Audible, and it appears to cost $15 per month, and include just one book per month. The cost, though not trivial would not necessarily be a deal-breaker, but the one book per month would be. If there is a similar service out there with an annual fee and a significantly higher book limit, that might be an option. We'll need to do some research. Again, it was only earlier today that we discovered this was going to be an issue, so we haven't done a lot of looking around yet.
I assume when the car's sound system is playing music from flash drives and a call is received, the music is paused, right? So if that is true, if we came up with a way to play audio books from a flash drive (instead of from a phone, with Overdrive) then presumably the audio books would also be paused during an incoming (or outgoing) call, right? If this is possible, this would offer the best solution.