It is a good reference, particularly for iOS. The book only includes GCD, Operations, and some concurrency problems. I think it would be good to have Modern Concurrency, NSLock, and some mentions for POSIX API where these frameworks sit. But make sense to read it to gain a brief perspective about concurrency.
This book is not the right place to start for this topic but instead is a refresher. I recommend reading several blogs and StackOverflow questions to get a better understanding.