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.
Yes, there is a BETA book (and eBook) you can already download right now: http://pragprog.com/titles/amiphd/iphone-sdk-development
With Apple shipping the iPhone 3.0 update the NDA is officially gone and they were able to ship an updated copy of the book. There are a ton of changes for 3.0 but here are some of the highlights. -Core Data -Map Kit -iPod Library Access -AFFoundation -Core Audio -Updates throughout for 3.0 API changes
The book's page over at pragprog.com has some new excerpts too in case you are still on the fence about purchasing you can see a bunch of the new stuff in these new excerpts. They are only a few weeks (6 to 8) away from the dead tree version now.