I\'ve looked through the site to make sure this isn\'t a duplicate. Apologies if I\'ve missed something.
I\'ve been doing iPhone development for about a year now, an
I started out with Mac OS development and went to iPhone development. You probably already know the core concepts about how the APIs work. The biggest problem I see developers who transition from iPhone to Mac is getting used all the subtle ways that Mac users expect their apps to work. Things like handling the Page Up and Page Down keys to scroll your view (shame on Tweetie for not supporting this). Or handling multiple windows. Or multiple display sizes (and making sure not to position windows off screen).
For the technical programming questions, Apple's own documentation is the best place to start. For the nuances of designing for the Mac OS and doing things right, there isn't a book or web site that has all the answers, as far as I know. Mostly that comes from using apps that are Apple Design Award winners and imitating what they do.