OK - I understand that I can download books from my Kindle account using the Kindle for iPhone/iPad App. But can I side-load books purchased from Amazon and stored on my iMac TO the iPad. And can I move books purchased from Amazon stored on my iPad TO the iMac for storage.
Or do I just have to store books on the Amazon Kindle website?
When you have content on Amazon, you can control what devices that it "pushes" to.
I have the iPad & iMac Apple Kindle clients, and I have no real Kindle. I have registered both devices with Amazon, so when I buy a Kindle book I have to push the books to both devices.
Then in the Kindle client when you fire it up you have to sync it with Amazon, and it will download your book. I believe the clients themselves have no capability to share the content inside them ... which is the point of the Kindle. The Kindle book data is not really accessible to the iPad in the way you are thinking.
Actually you can side load a .mobi or .prc file to the Kindle app on the iPad, iPhone or iPod Touch using iPhone Explorer. Just place the file in
Device/Apps/Kindle/Documents
and it will show up after restarting the Kindle app. The .mobi or .prc file must be DRM free.