I registered these two 'devices' as Kindles in my Amazon Kindle account, and now when I buy a book I have to 'send' it to both devices where I can read it on either one, and it will track where I am on both devices as well. In addition to loading my iPad with the Kindle device, I found there was a Mac Kindle client as well, and since I have a 24' iMac, I downloaded it as well from the Amazon. You sign in and buy whatever you want, and then sync it to the desired devices. You can goto the Kindle store by clicking that in the Kindle app, which then starts the Safari browser app and goes to Amazon.Com's Kindle store. when you start it up, it is just mostly just like the interface to the Kindle machine. I searched for the Kindle App in the app store, and downloaded it to my iPad. I have just done this in the last few days.
