Learning iOS Design is a book written by William van Hecke, the User Experience Lead at the Omni Group (the makers of OmniFocus and other well-known apps).

In this book, William takes you through the process of becoming an iOS app designer, regardless of your experience level. You'll learn how to flesh out your app ideas, from early sketches and wireframes to full-on prototypes.

The book features a foreword written by Lukas Mathis of Ignore the Code, and contains enough content that it's essentially an app design textbook. It costs $30 on Amazon and $36 on the iBookstore, which could easily pay for itself in the long run once you get out there and make some amazing apps.