Amazon should allow LIMITED sharing between Kindle owners -- say three times for each download. So my father can share a book he downloaded on his Kindle with my two brothers and I, and we could share books we downloaded with him.
Right now, when we read a book we love, we pass it around. We don't all go out and buy our own copy. But I'll tell you what...my brother and I would buy our own Kindles (Dad has one) if we could share books faster than snail mail!
Sure might boost sales in this economy, too.