I got into Mac OS X / Cocoa / iPhone programming after reading the wonderful book by Aaron Hillegass called Cocoa Programming for Mac OS X and always thought that Big Nerd Ranch should do an iPhone book and, they have. I’ve had a flick through it and it looks like an excellent read. It’s called iPhone Programming: The Big Nerd Ranch Guide. On the outset it looks to be a similar style to Cocoa Progamming for Mac OS X in that it talks about Objective-C and things you need to know there such as memory management and then goes on to describe the various parts of the iPhone SDK.
Anyway, I’d recommend buying it if you’re thinking about getting into iPhone programming.