Of course this book is so valid when iOS 5 is out.
iOS 5 isn't iOS being written from the scratch, it is built upon iOS 4. You can check what is new in iOS 5 and learn them by yourself, shouldn't be that difficult.
There won't be major change in the APIs between iOS 4 & iOS 5. Framework may get new APIs or existing APIs may get changed, but chances for version specific framework are so less.
I am working since when iOS 2.2 in the market. I have never face major difficulties due to API differences.