There’s another book available from Apple, though it’s not yet in the iBookstore. Using Swift with Cocoa and Objective-C gives practical information for calling Cocoa APIs from Swift and incorporating Swift into existing XCode projects, possibly alongside legacy Objective-C code.
There is also a section on porting Objective-C code to Swift, strongly suggesting that Apple is positioning Swift as a core part of the future of its development platform.
For now, the book can be downloaded from Apple as a PDF here.
