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.
There are tons of websites that can offer you some help in beginning to learn Cocoa. One is the Mac Developer Network which hosts a podcast called "Late Night Cocoa" [iTunes Podcast Link] -- this is definitely worth a listen to.
If you have joined the $99 iPhone developer program, then you might want to check out the iPhone Developer Forums (currently in "beta" and found when you log into the Dev Center under "Development Resources").
Other great places to find information include: Stack Overflow and Cocoa Builder. These two sites allow you to find answers to specific questions that you might have while you are developing your apps. You can also find answers to the occasional question just by searching Google ... there are a lot of great resources out there, and we've only covered a few.
Of course, one of the best places to visit is the Apple documentation. Not only is it filled with pages and pages of information helpful for anyone who needs to look up APIs, classes, methods, etc., but you will use it on almost a daily basis. In addition to their iPhone Getting Started guides, they have an iPhone Reference Library available right at your fingertips, along with sample code.
There are other great resources out there, and I definitely don't know about all of them, nor would I be able to include all of them in a post. If you use another valuable site, book, or forum, be sure to let us know in the comments!