The Swift language is developed by Apple, and a number of documents are available from the Swift developer page at https://developer.apple.com/swift/. This includes a language reference guide and an introduction to the standard library:
The Swift programming language: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/
The Swift standard library reference: https://developer.apple.com/library/ios/documentation/General/Reference/SwiftStandardLibraryReference/
Integrating Swift and Cocoa: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/BuildingCocoaApps/
Swifter provides a list of all the Swift functions: http://swifter.natecook.com