This is cool.
One of the books on my Kindle is Battle Cry of Freedom, a wonderful history of the Civil War that Daniel recommended to me highly. I just got a notice from Amazon that that edition has been updated; the e-mail included instructions for getting the update on my Kindle.
No, the ending didn't change. The maps were updated. Those maps were the one thing that was poor about that e-book edition. I don't know if I'll ever reread the book, but if I do, or if I look into it for reference, I'll have the better maps.
Some people have said that this changeability is a drawback to e-books. To my mind, it’s one of their many strengths.