The books can be copied, but they can't be read on another reader or even on another Kindle for PC app due to DRM. They can only be read on that particular machine. To read them on another machine you'd have to install the app there and use your Amazon account to send it to that machine in order to reset the DRM.
Alternately, you can strip the DRM in order to free them and use any reader or application that can read mobi files (as long as the books are mobi and not Topaz).
Please remember, though, to respect copyright. Most people strip DRM so that they can use their legally purchased books (well, Amazon claims you purchase them, not license them) anywhere they want, and not to pirate.
There are threads on MR that should help with the DRM stripping.
Hope this helps.