TL;DR: I've already developed some projects in Objective-C 2 years ago. After that, I start to develop all of my projects in Swift because this was the very hot trend in iOS development community. I don't just only remembered the old skills that I learn but also I learned new things.
As an iOS developer knowledge of Objective-C is not a plus, it is a requirement. We don't work best architected, clean projects all time. Sometimes codebase comes from 5 years ago, sometimes some features don't exist in Swift. This book easily provides that requirement to developers who already know Swift.
The book has written with clean paragraphs and direct style. If you want to learn just how to code Objective-C swiftly without fundamentals and history, this book for you! This was the thing that I'm looking for.
Let me preface this by saying that I really enjoy the author's work in general, and wish him the best on all his future endeavors.
This book is decent. It somewhat succeeds in its mission in that it will help you if you are used to programming in Swift to grasp some Objective-C concepts. The author includes lots of history regarding Objective-C and how it shaped the Swift language which is very interesting.
The downsides are that there are quite a few errors and misleading parts in the book, which is understandable considering the rate that the author is putting out books. I managed to work my way through the mistakes and comprehend the material the author was trying to impart so it wasn't an entirely big deal.
This book contains lots of concept and theory and not much applicable material. The "exercises" given seem to have been swept in as an afterthought. There are only 2 projects you work on (3 if you include the video) and they are extremely trivial projects. Some concepts are extremely fleshed out and then some are glanced over in a single paragraph. The book feels very inconsistent at times and lacks a certain degree of focus.
Reading this book will not make you a capable Objective-C developer by any means and I feel like there are much better books out there on the subject although this one does provide a unique perspective for Swift developers.
It explains the basics of Objective-C thoroughly, and covers material that I didn't know even after programming in Objective-C for five years (half a decade and I didn't know all the property qualifier keywords!)... so I'd recommend it as filling holes for people who had to learn Objective-C on the fly, for practical purposes.
The main issue I have with the text is the underlying tone; through snide offhand remarks like "eek!" and the occasional mention of teams, it seems to assume that Swift is the better -- or more advanced -- language, and that Objective-C is some foreign entity that you may encounter if and only if you embed into a larger team of veteran developers. The discussion of auditing code for nullability and the frequency at which you may encounter certain terms seems to assume that you are (or should be) programming in Swift, and that certain Objective-C mechanisms surface only in existing (legacy) codebases. I would have preferred to read without those assumptions, i.e. that I may wish to write this Objective-C code for a new project from scratch, and think of concepts from a code creator's standpoint rather than a code modifier (or code cleaner)'s standpoint. Objective-C is a very powerful language if you know how to use it, and I think the text doesn't always do that perspective justice. It's still a very good text.