How do you move books from one Kindle to another?

On one kindle you take the books you want to move, sync them to iTunes, and down load them on the other kindle. I have two Kindles and when I read a book on either one it asks me if I want to go to the place I left off on the other Kindle, I gues t uses the cloud to do this. When I get a book from the Kindle store it shows up on both Kiindles, it must be magic.