If you bought your ebooks at Sony or Kobo(or any number of other online sites) there will usually be some sort of copy-protection, which would ordinarily prevent you from sharing. This "protection" is usually referred to as "DRM".
Some people think DRM is OK, but lots and lots of people think it's more of a rip-off.
Regardless, it is possible and even quite easy to commute your bought and payed for ebooks to other ereaders, which is only fair since an ereader only lasts a couple of years and your next model might not be from the same company. In essence this is the same as "sharing" or letting somone "borrow" your ebooks, which, again, is only fair since poeple do it with real books all the time.
Many people(myself included) consider the freeware program "Calibre" the best way to take care of the problem. You'll have to learn how to use the program and add "plugins" to it make it work.
But there is a really easy way, ePUBee(also freeware)! It's a three step "drag&drop" method and all my technophobic acquaintances are using it.
Here's what a blogger I respect has to say about the whole thing:
http://www.the-digital-reader.com/20...y-against-drm/