Here's a link to Mark Levine's long-but-worthwhile piece about car-sharing and the car-sharing business. I've read several explanations of how car-sharing works, but they invariably read like computer owners manuals; I never make it all the way through.
This piece too spends a good bit of its middle on the business aspects of sharing cars. I managed to get through that part okay and enjoy the last page or two. Worth reading to the end, I'd say.