I wouldn't hesitate, as a Kindle 2 owner, to call up Amazon or drop their support e-mail a note (easy to find at
www.kindle.com) with the serial number (it's on the back of the unit). They will find a way to return it to the owner ... provided it was purchased from Amazon.
Since you know it has five books on it, you must have been able to turn it on; and it must have been registered at some point with Amazon.
In future, this might get more difficult as a brand new unit could be purchased from a store like Target and not yet registered. In that case, Amazon would not be able to trace it.