And it's quite a good manual at that. Starts with a high-level overview of language features in plain English, then provides a firehose of nitty-gritty.

Here's a link to the iBook on iTunes: https://itunes.apple.com/us/book/the-swift-programming-langu...

And the official e-book is available in multiple formats (PDF, MOBI, ePUB) on http://swiftlang.eu