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:
- the canonical cocoa-dev mailing list run by Apple (which, unfortunately, has only the woefully lame web access implemented by Apple, and sporadically augmented by unreliable third-party efforts, making it mostly useful via actually subscribing to it from one of your email acccounts)
- the CocoaDev wiki
- Rogue Amoeba's Mike Ash has an unusually useful Q&A Fridays series on his blog
- the advanced Mac programming book from Aaron Hillegass, Advanced Mac OS X Programming
- although Apple's docs are pretty good now, in the bad old "description forthcoming" days, a lot of us learned how Mac OS X ticked from open-source code published by reputable software houses, like Omni's frameworks which you are free to embed as-is into your apps, or cherrypick code from
(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.)