Inside Macintosh Volumes I-III are very useful. I'd recommend them since you can get them cheap. Search for them on Amazon. Volumes I-III can be found combined in one large hardback. I think it cost me less than $7 including shipping, so you might as well get it.
The other book I would look for would be either of the "Macintosh Programming Primers" written by Dave Mark and Cartwright Reed. There's one for Pascal and one for C.
For early macs, pascal was the language of choice. Inside Macintosh has it's code examples in pascal. That doesn't mean you have to go with it.
I agree that Think Pascal and Think C are the way to go.
So your target is a 512k mac right? What will you be developing on? I develop on a System 9.x iBook, and use Think Pascal 4.x. It works pretty well. I find a few differences when I run the app on the iBook vs the System 6 SE, but I apple-talk the two when I'm at home, and it's easy to test on each.
If you're developing on the 512k, I don't know if that will work. Maybe you'll have to use version 3.
I've found Think Pascal 4.x on the web. I forget where, but it's around.
Ok, that's enough rambling. If you need anything, ask. I'm no pro, but I love developing for these old macs.
Nathan