Some books are just invaluable and couldn't be replaced with another. Aaron Hillegass' Cocoa Programming for Mac is just that book. Currently in its 3rd edition, the book gives you much of the Cocoa programming information that you need to program for both the Mac and iPhone. There are only a few subtle differences in programming for these platforms, namely the use of the Cocoa Touch. If you ever have the chance, going to one of the Big Nerd Ranch Cocoa programming classes gives you the ability to learn Cocoa hands-on.
Another title that is useful to beginning iPhone developers is the Beginning iPhone Development book. This book has a useful approach to stepping into the world that is programming on iPhone. It talks about numerous topics including UI design, Quartz, and OpenGL. Also covered in the book are APIs like CoreLocation and interfacing with the camera.
If you already know Cocoa and a little about iPhone development, Erica Sadun's iPhone Developer Cookbook is a great jumping off point to start development. She assumes, however, that you already understand Cocoa.
Continue reading to learn about more valuable books, websites, and resources for iPhone/Mac developers.