Ask HN: Experienced programmer, beginner to iOS (objective-C). Where to begin?

What is the quickest way for an experienced programmer to come up to speed on iOS? Looking for recommendations on a good book, tutorials. Pointers to good repositories on github also appreciated. I have started reading open-source code from some apps. I want to write an app which needs custom UI elements (going beyond XCode's available UI elements). An example: a flipboard like app. No game dev. - I am comfortable in C, C#, Python. I can write Javascript when needed for my web-apps. - NO experience with Objective-C. Fairly comfortable with ANSI-C. - I have written web-apps: HTML, CSS, JS as needed. - I have written desktop apps: So, I am familiar with how to use available UI elements with code behind. I see clutch.io, RubyMotion, MonoTouch as options too. Any feedback from folks on these?