https://www.amazon.com/Mac-OS-iOS-Internals-Programmer/dp/11...
Basically the C, C++ and Objective-C stuff, nowadays Swift as well, completly unrelated to UNIX.
https://www.amazon.com/Mac-OS-Internals-Approach-paperback/d...
https://www.amazon.com/MacOS-iOS-Internals-User-Mode/dp/0991...
https://www.amazon.com/MacOS-iOS-Internals-II-Kernel/dp/0991...
https://www.amazon.com/MacOS-iOS-Internals-III-Insecurity/dp...
https://www.amazon.com/Mac-OS-iOS-Internals-Apples/dp/111805...
However the future leads to userspace extensions, so their usefulness is kind of relative, beyond targeting pre-Catalina versions.
http://www.nextop.de/NeXTstep_3.3_Developer_Documentation/
https://www.amazon.com/Mac-OS-Internals-Approach-paperback/d...
https://www.amazon.com/Mac-OS-iOS-Internals-Apples/dp/111805...
Technically it is kind of the same, but with different set of configured features.
Already a bit oldie,
"Mac OS X and iOS Internals: To the Apple's Core"
https://www.amazon.com/Mac-OS-iOS-Internals-Apples/dp/111805...