The iOS Design Guidelines - Ivo Mynttinen / User Interface Designer →
Designing iOS apps can be difficult sometimes, but finding correct and up-to-date information about all of Apples’ devices shouldn’t be. These design guidelines will help any designer who’s building neat things for iOS get started within seconds.
I found this guide really really useful. This isn’t an Apple official guide, but it’s really precise, and keep all relevant information (and some good design thoughts) in a quickly readable form.