There are no books available yet for the iPhone SDK, as it is still under NDA (why, I have no idea!)
That said, when you download the SDK, you receive a lot of good documentation direct from Apple - tutorials, programming guides, API references etc. It's a bit more work then just picking up a book, but it's not bad.
There is probably one book from the Mac world that is worth looking at - I forget the title, but it's written by Aaron Hillegass - everyone just calls it the Hillegass book... The first half of that book just teaches you basic Cocoa patterns, that are every bit as applicable in the iPhone world as they are in the Mac world. Once you have got those basics nailed down, the Apple documentation does become a bit easier to use.