This! I remember reading 'Cocoa Programming for Mac OS X' by Aaron Hillegass and being fascinated by it. Coming from Java, C, doing some DSP code, web work, Objective C and Cocoa at first seemed just utterly alien and a bit wrong.

Message passing, these loosely coupled 'delegates', a mixture of hardcore C, needing to do memory management but also just passing objects around in the runtime... messy, weird... but in the end the right solution to make flexible but performant software. I learned a lot from it. I should dive into the history of NeXT one day.