The iPhone SDK 3.0 has been in beta for a few months, so I would think that by now there would be some good reference/tutorial material floating around in the form of e-books (it may be a little too early for print books).
So far, I only came across iPhone SDK Development from The Pragmatic Bookshelf. It covers the SDK up to version 3.0, but the book is still in “beta”.
Are there any others?
Cocoa Touch for iPhone OS 3.0 will be released soon. The author is a prominent member of the mac development community in the Phoenix area, and has been working hard on this book in addition to creating and updating apps for the AppStore.