If you have any interest in user interface design, Apple’s Human Interface Guidelines is an interesting document to peruse, if for no other reason to get a sense of why Apple makes the design decisions that it does. And you can now get it through iBooks.
The 20MB e-book contains the full Human Interface Guidelines for iOS 7, complete with illustrations and annotations. And although it’s geared toward developers, it’s written in clear, plain English so anyone familiar with iOS should be able to get an idea of what’s going on. Oh, and it’s free. Head over to the iBooks Store to get your copy.