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
Thankfully, Cocoa on the desktop and Cocoa Touch are very similar. I would recommend just downloading some beginner sample code from Apple's website. Since you already know Objective-C/Cocoa, you should be able to follow along pretty easily. If some parts are confusing, you can skim through Apple's guides.
Things that are the same:
Things that are different:
Edit: Overall, the main different the iPhone has more 'pre-built' UI classes (UINavigationController, UITabBarController, etc.) you can use in your app to make it look nice and pretty (and most importantly, consistent with Apple apps).
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.
While the iPhone development evironment isn't strictly a subset of the Mac, since there are some improved and iPhone-specific bits, it is nearly so (and it's mainly the good bits). Thus, the Mac is probably the easiest platform to move to from iPhone.
On the Mac, you will especially want to learn about Cocoa Bindings, which underly a lot of modern UI work, but aren't (yet) on the phone.
Addressing your specific question, some well-respected free Mac coding resources include:
(Hmm, I don't usually post here, so I was prohibited from hyperlinking the above references... can I put them in as text? Uh, nope! Well, there's always google, I guess.)