Some time ago I reviewed iPhone in Action from Manning. While there are other "starter" books out there, if you know how to code and you want to get up-to-speed on everything from web apps to the SDK, this book is a great primer. Of course, the pending 3.0 update to the iPhone's OS will add many new features, and the book's authors are hard at work with an update that I'm told will be available as a downloadable e-chapter. The authors will first serialize it on their blog, so you can keep up there as well.
Until the update is ready the good folks at Manning have provided a code that'll get you 50% off iPhone in Action until April 30, 2009. To get the discount, just go to the site and when you order use this code: tuaw50. Remember, this is only for the current edition of iPhone in Action.