The Swift Programming Language
https://itunes.apple.com/us/book/id881256329
Apple Developer
https://developer.apple.com/swift/
Welcome to Swift
https://developer.apple.com/library/prerelease/ios/referencelibrary/GettingStarted/LandingPage/
iBooks: The Swift Programming Language
https://itunes.apple.com/jp/book/swift-programming-language/id881256329?mt=11
書籍の形にはなってないけどドキュメント
The Swift Programming Language
https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/index.html#//apple_ref/doc/uid/TP40014097
Using Swift with Cocoa and Objective-C
https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/index.html#//apple_ref/doc/uid/TP40014097
Swift Standard Library Reference
https://developer.apple.com/library/prerelease/ios/documentation/General/Reference/SwiftStandardLibraryReference/
Swift Blog
https://developer.apple.com/swift/blog/
Resources (Xcode 6が無料でダウンロードできる)
https://developer.apple.com/swift/resources/
Wikipedia Swift (プログラミング言語)
https://ja.wikipedia.org/wiki/Swift_(%E3%83%97%E3%83%AD%E3%82%B0%E3%83%A9%E3%83%9F%E3%83%B3%E3%82%B0%E8%A8%80%E8%AA%9E)