iOS 10 Programming Fundamentals with Swift It complements apple's offical swift documentation well. If you're looking for something at a similar technical level as the apple documentation but explained somewhat differently that's a good book.
Assuming you already know a few languages, the one thing that I think deserves the most attention are optionals since they're a relatively new feature in programming languages. Most of the other stuff is nothing new. Also, structs and the fact that the most basic data types are implemented as structs.
very realistically. is started learning swift in may, beginning with this book (german):
https://itunes.apple.com/de/book/einf%C3%BChrung-in-die-app-entwicklung-mit-swift/id1215105506?mt=11
the english verison ist 5x longer, so definetly more in depth:
https://itunes.apple.com/de/book/intro-to-app-development-with-swift/id1118575552?mt=11
the books are free and made by apple. they come with excercises that helped me a lot. after i finished the book (1-2months) i imidietly started with my idea and quickly noticed that my skills arent good enough yet.
i ordered this book:
https://www.amazon.de/gp/product/1491970073/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
its super great, extremely well written and it answered so many questions that apple didnt.
yesterday i enrolled in the apple developer program and im planning to submit my first app to the appstore end of this month.
the app uses firebase, pushnotifications, a timer, has a tutorial (when opened for the first time) and some more functions.
definetly get started! swift is fun!
also connect to developers in your area through meetups for example. they are always happy to help!