Safari Books Online is a digital library providing on-demand subscription access to thousands of learning resources.
Apple’s developer websites and mailing lists are a primary source of information: http://developer.apple.com. Unfortunately, Apple tends to write additional documents or tech notes on a subject rather than integrating the information together and clearly targeting it. Fortunately, Apple is not the sole repository of information.
The http://www.cocoadev.com website is a repository of information about Mac Cocoa. In many ways, Cocoa Touch is similar, and its advice is pertinent.