Effective Objective-C 2.0: 52 Specific Ways to Improve Your iOS and OS X Programs (Effective Software Development Series) by Matt Galloway
It took me pretty long time to finally finish this book. I can say that it is good source of effective practices and hints about Objective-C and its usage.
You can argue why I am reading books about Objective-C when Swift is around. Short answer is that most of iOS development is not the language but libraries and how to use them. I want to learn that from proven books rather than thinking is Swift changed since this blog post or is this error in the blog post? Swift is still evolving and I do not want to be guinea pig, especially when I don't really know how various things match app development.
Objective-C was not the only language used for iOS development. Many libraries are available only as C libraries and I do not think it will change soon. Learning some Objective-C and C for sure will be worth it.
As of the book itself. Sometimes it was too hard for w newbie like me to understand everything but most of the time it helps to read "iOS Programming" at the same time to gain more context.