Also, the Objective-C Runtime reference[1]. About a decade ago, I learnt Objective-C in a day, by reading "The Objective-C Programming Language" and the Runtime Reference. It's such a beautiful and simple language, if you know C. I wish Swift were as simple and succinct. AFAIK, Objective-C++ is still the easiest way to bridge C++ code with Swift code. Knowledge of Objective-C is still relevant for anyone building apps that need to use C++ libraries.

[1] https://developer.apple.com/documentation/objectivec/objecti...