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Well, the short answer is that the book is completely updated for iOS 9, Xcode 7, and Swift 2. We don't have a list of changes anywhere that I could point you to, though.
We do have a upgrade discount path. If you have an older edition, just register that book in your oreilly.com account by going to the Register Print Books page and then go to the Your Products page of your account and click on the registered book and you should see the upgrade discount button to get that deal.
It's 40% of the newer edition print book, or 50% off the ebook.
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