Is it as in depth as OP appears to be looking for? I've found Paul's content to be good for quick references to high level concepts, and brief how-tos for beginners, but I've not found it to be good for deeper explorations. That said, I haven't read this book, only his free content.

Good question. It, like all of Paul's books, is more of a series of tutorials than a reference.

I would say it is a decent place to learn how to get started with macOS app developing using either SwiftUI or AppKit. For somebody with no knowledge of macOS app development it will put you in a good position to be able to work from Apple's documentation and further learn the platform.

However it is not anywhere near as in depth as something like Petzold's Windows programming books. Although it isn't meant to be anything like Petzold's books so comparing is kinda pointless anyway :)

I bought Paul's books in a decent value (to me) sale a few years ago and I rate them quite highly overall. Much higher than most other self-published programming books.