"You can buy music from Amazon. You can load that into iTunes. You can then play it on your iPhone/iPad/iPod. Why can't you do the same for Kindle (etc) books?"
When you buy music from Amazon, it's in MP3, a format that iTunes supports natively. For ebooks, Amazon uses the proprietary Mobipocket format, which only their Kindle app can read. Apple's ebook reader app uses ePub, as do most other ebook resellers, like Barnes&Noble. That's why ebooks bought through Barnes&Noble work just fine in Apple's iBooks.
When you buy music from Amazon, it's in MP3, a format that iTunes supports natively. For ebooks, Amazon uses the proprietary Mobipocket format, which only their Kindle app can read. Apple's ebook reader app uses ePub, as do most other ebook resellers, like Barnes&Noble. That's why ebooks bought through Barnes&Noble work just fine in Apple's iBooks.