This is a curated list of reference material that anyone who develops in the Apple domain will hopefully find helpful. Most items are things I have used and can vouch for.
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The Swift Programming Language
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Foundation
UIKit
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Coordinators - Soroush Khanlou
Patterns to Destroy Massive View Controllers - Soroush Khanlou
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Advanced Debugging - WWDC 2018
Dancing in the Debugger - objc.io
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CI
CD
Analytics & Crash Reporting
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Build Configuration Files (xcconfig) - NSHipster
Build Configurations - Thoughbot
Conditional Compilation - Dave Delong
Xcode Build Settings Reference
Xcode Build Variables List - @gdavis
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Current Apple Documentation Archive
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How to Deliver Constructive Feedback in Difficult Situations
iOS Factor (Twelve Factor Apps)
Technical Debt - Martin Fowler
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Apple
Tools and Blogs