Apple has published iOS Human Interface Guidelines as a free book on the company's iBooks book store. As noted by MacStories, the guidelines were previously available online only through Apple's developer portal, but are now downloadable by anyone.
Sample from iOS Human Interface Guidelines
Apple's description of the book says simply, "Create a superlative user experience that's founded on Apple's design principles and guidelines." Despite that minimalist description, the book contains everything a developer needs to know to create iOS apps that meet Apple's human interface requirements.
Topics include animation, in-app purchases, design principles (as shown in the screenshot above), typography, Passbook, and many more over 41 chapters.
The book is officially an "Enhanced" title that includes videos.