Apple has added the iOS Human Interface Guidelines for App Store app designers to the iBookstore. Previously, the guidelines were solely available from the online Apple developer portal, and the addition to the iBookstore makes the guidelines for designing iOS 7 apps more easily accessible.
Federico Viticci notes at MacStories that the book is properly optimized for viewing on the iPad:
The 20 MB guide is compatible with iPads as well as Macs running iBooks on OS X Mavericks, and it takes advantage of the app with inline video playback, two-page page layouts, and built-in annotations (plus, of course, font size and color controls for reading settings).
The book can be downloaded for free on iOS or OS X.