There is the official Swift Book: The Swift Programming Language by Apple Inc.

Book Link: https://itunes.apple.com/us/book/the-swift-programming-langu...

Swift is a new programming language for creating iOS and OS X apps.

Swift builds on the best of C and Objective-C, without the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to make programming easier, more flexible, and more fun. Swift’s clean slate, backed by the mature and much-loved Cocoa and Cocoa Touch frameworks, is an opportunity to reimagine how software development works.

This book provides:

- A tour of the language.

- A detailed guide delving into each language feature.

- A formal reference for the language.

Hope it helps.