Scott Jensen has released a beautiful iBook on the basics of iPhone UI design. This is not a pixel-pushing, put-the-gradient-here book, it's all about the fundamentals of design, getting the flow and the workings right.
From creating icons to providing user support, iPhone Blueprints covers the start to finish of user interface design. The chapters are rich with examples, and resources for that demonstrate overarching principles for creating rich and effective interfaces.
It has a number of interactive elements, including an Adobe Illistrator icon template and a number of case studies.
I consider this an absolute must read for anyone doing iOS development. It's not especially long, it's free (not anymore - $1.99), and the content is excellent. I love the layout, too. Lovely work.
It's available for free from iTunes, or search for it in iBooks.