You're correct that the copyright notice is no longer required under the Berne Convention. But it's still a good idea to include it, as you can't get statutory damages for infringement if the work doesn't display the notice.
If the work was first published in 2006, then that is the copyright date. If the work was revised in any way for Kindle, then you might want to include both dates, thus:
(c) 2006, 2013
If it's really important to you, I suggest talking to a lawyer who specializes in intellectual property law.