The problem specifically with books is more that developer attention is split between Catalyst (UIKit), SwiftUI, and (AppKit | UIKit), and if you’re going to document something in this space, everything that isn’t AppKit has a larger audience.
But I do need to point out that AppKit is now in the position Carbon was in during the 2000’s — the future on macOS, if there is one, is evidently UIKit via Catalyst or SwiftUI no matter what assurances Apple makes. I would not advise a new developer to learn AppKit at this point in time.