Whenever possible, add a realistic, physical dimension to your application. The more true to life your application looks and behaves, the easier it is for people to understand how it works and the more they enjoy using it.
It’s a small detail, but it certainly reinforces the type of computing environment the iPad has: more abstracted than the usual PC/Mac/Unix operating systems and its desktop metaphor.
iA is none too pleased with this guideline, however, as the eyecandy enter kitsch territory and fails to solve interaction problems.
Using a book shelf for choosing a book is a consistent metaphor, but does it help you understanding the interface? Does it help in any way? Do you still like the hard to scan bookshelf with after looking at it for the 200th time? What if you have 200 books?
Note: I’m not saying that using quirky metaphors and plastering your app with special effects will kill the perspective to sell your app. Unfortunately the audience for kitsch is a bigger audience than the elitist UID guild. But usually phony design doesn’t have a long life span.
Metaphors should not distract or confuse, they must clarify. Apple and those that design for it love their beautiful interfaces and the metaphors that inspired it, and while attention to detail is highly admired, design abuse via mainstream device iPad, will become rampant. It’s tempting to pull out all the stops, but as with many things in life, exercise restraint.