There are less radical changes to the API, that’s true. Auto-Layout it probably the biggest change. The rest are additions that you can check out later, like Pass Kit, Reminders API, UICollectionView or better social integration.
I wouldn’t worry too much about an iOS 5 book being out-dated, if it’s good. Make sure it teaches ARC and don’t spend too much time learning autoresizing masks (not that it would be very complicated, but Auto-Layout is a better choice now).
solved Is it okay to be learning to program for iOS with an iOS 5 book? [closed]