A book I heard about via Matt Gemmell is "Learning iPad Programming" (Kirby Turner, Tom Harrington) - it's really an excellent overview, explained by programming a real-life iPad app step by step. Massive coverage of tons of major topics. http://www.amazon.co.uk/gp/product/0321750403

Also, as others mention, the Stanford lectures are awesome, especially combined with the iTunesU iPad app.

And finally, for me the breakthrough into iOS programming came when I went to a hackathon and pair programmed with a total iOS pro, I learned WAY more in a few hours than I did reading a bunch of tutorials.