Big Nerd Ranch books
The books written by the people at the Big Nerd Ranch have been excellent resources for Cocoa development especially the 'Cocoa Programming for OS X' book by Aaron Hillegass. Other books include 'Objective-C Programming', 'Swift Programming' and 'iOS Programming'.
objc.io books
The geeks at objc.io have been writing excellent books on Advanced Swift and Core Data.
objc.io books: App Architecture
The guys from objc.io have recently published a new book: App Architecture – iOS Application Patterns in Swift.They discuss and compare five major application design patterns … a book for developers that want to bring some structure in their code.
NSHipster books
NSHipster has been providing excellent articles about cocoa-development even for non-hipsters. You can now enjoy this through the Obscure Topics books for Objective-C and Swift as well as their low-level book CFHipsterRef and, obviously, the Fake Book.
The Complete Friday Q&A
Michael Ash's Friday Q&A has been been a fountain of low-level knowledge for years and you can now get all of it in book form!
Cocoa Design Patterns
"Cocoa Design Patterns" by Erik Buck explains how Cocoa development is done, mostly for those coming from other platforms.
Mac OS X Internals: A Systems Approach
"Mac OS X Internals" by Amith Singh is the most comprehensive investigation of the technical underpinnings of macOS and iOS you could ever wish for. However, as the name suggests, its a bit dated by now.
Objective-C Pocket Reference
Objective-C is a simple language that can be learned in an afternoon. The O'Reilly Pocket Reference is all you could ever need. Sadly it doesn't cover anything beyond Objective-C v1.
Amazon Hot New Cocoa
Not a book itself, but Amazon's category for hot & new apple-development books.