This a resource library of the Apple developer resources that I have regularly accessed. It will continue to grow as my path to learn app development continues.
App Guidelines - https://developer.apple.com/library/content/qa/qa1686/_index.html
Human Interface Guidelines - https://developer.apple.com/ios/human-interface-guidelines/overview/themes/
API reference - https://developer.apple.com/documentation
Apple Developer Forum - https://forums.developer.apple.com/
Apple Guides and Sample Code - https://developer.apple.com/library/content/navigation/
Catching errors and what they mean
OSStatus - https://www.osstatus.com