Apple has put its iOS Human Interface Guidelines (HIG) on the iBookstore as a free book. The book, which you can read in iBooks on an iOS device with iBooks 1.5 and iOS 4.3.3 or higher, as well as any Mac running Mavericks, contains 228 pages, and provides guidelines for everything from app layout and color to working with trademarks. Short videos and animations demonstrate different aspects of app design, such as the effects of a button press or pinch-to-zoom.
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