I've always like the idea of a Netflix for books but it doesn't seem profitable to me. USPS Media Mail would be the only way to ship them. Right now from DC to Pittsburgh the cost of shipping a 1lb book is $2.23 and the max is 10lb for $5.38. Even then that's assuming you have regional shipping centers for these things all over the country, or you charge customers different rates depending on how far away they are from the shipping center.

It's too much work and I don't think it's profitable in the long run.