Apple has published its “iOS Human Interface Guidelines” for iOS 7 as a free ebook on the iBookstore. Previously, it was only available for registered Apple developers.
The book is designed to help developers “create a superlative user experience that’s founded on Apple’s design principles and guidelines.” To view the book you must have have an iOS device with iBooks 1.5 or later and iOS 4.3.3 or later, or a Mac with iBooks 1.0 or later and OS X 10.9 or later.