The key difference is that a physical copy of a book can only be loaned out to one person at a time, so it is in no way violating copyright.  No copy is made.  A lot of people will break down and buy a book that they really want to read if there's a long waiting list at the library (I've done this plenty of times).  Once you cross over into the digital realm, you're talking about copying and distributing.<p>Unfortunately, I don't know what the digital equivalent of the library would be.