At 268 pages, this book is shorter than many programming books.
It describes developing for an open (jailbroken) iPhone. After the first edition sold out, this is the version with minor updates for the iPhone 2.x firmware. This book teaches you about the iPhone APIs used by the built-in Apple applications, but you should be aware that it does NOT target the Apple iPhone SDK, and does NOT guide you in developing apps for the AppStore, though the code will generally be applicable for AppStore applications.
It begins with a description of the process of jailbreaking, getting the compiler set up either on the Mac (hard) or the iPhone itself (trivial: http://soi.kd6.us/2008/09/27/so-i-made-my-iphone-say-hello-world/) and an introduction to Objective-C.
This book presents many complete example programs using the various iPhone UIKit controls, and presents information on Quartz (2d graphics) and the sound libraries.
It does not describe OpenGL ES (for high-performance/3D graphics) or web applications and APIs.
I found occasional editing errors - more than I'd expect in an O'Reilly Second Edition, ranging from typos (Quarts instead of Quartz) to old text describing an updated code example, to copy-and-paste errors between similar sections. Nothing too egregious, but distracting.
I list this book and other books that target the SDK in my Amazon Store: http://astore.amazon.com/iaw-20