I have read this book in Swift already, and I have read some other books from here as well. I think I have good enough experience on Swift, and now I am considering learning Objective-C.
Is the old version too old or have any other problem if I want to use it to learning Objective-C?
While the older version may be in Objective-C, the problem is that you are working through examples that also use an older version of iOS. This creates at least two problems:
The libraries that you are using may be deprecated, and have alternatives in Objective-C for you to use.
The version of Xcode that you are using now is going to be much different than the one in the older version of the book, which means navigation, and debugging will be different.
Finding support for when you get stuck might also be difficult.
I would say that for simpler examples, the older books might be useful to learn Objective-C, but if you are already confident as a Swift developer, I would suggest the following:
Focus on building your Swift skills further, since these skills are much more marketable, and this is where technology is trending. You can do this by looking into more advanced concepts like Swift 5, working with the Codeable Protocol, Concurrency, AutoLayout in code, etc.
You might want to look into working with different API’s in iOS like ARKit or CoreML.
If you absolutely want to learn Objective-C, I would also suggest that you challenge yourself by going through the current book, and try working through the chapters by doing everything in Objective-C All of the new API’s have Objective-C equivalents, which would be one way for you to learn Objective-C, while using the latest API’s that Apple has released, which allows you to essentially kill two birds with one stone