Apple's Developer Tools
These usually come with your Macintosh, either as a separate "Developer Tools" CD, or as an installer package that is on your hard-disk. Just double-click to install the developer tools, which includes Project builder (or xCode), and a /Developer folder full of samples, tools and documentation.
If you don't have them, you can register for a free ADC Online account on Apple's developer web site and download the Dev Tools from there.
Aqua Human Interface Guidelines
This is a very important document that gives you an overview over how Macintosh programs should behave and look, and introduces you to the terminology used in the other books.
Especially read anything that concerns itself with Events, Windows, Menus, Controls or Views. You will also want to read the File Manager, Navigation Services and Alias Manager docs.
The Mac OS 7 through 9 docs
After you've read the porting guide, you'll be able to read these slightly dated docs to fill in the blanks that the Carbon docs still leave these days. These docs (especially the "Inside Macintosh" suite of books) are very well-written.
In particular, I suggest reading "IM: Overview", "IM: Mac Toolbox Essentials", "IM: Files" and skimming "IM: Imaging with Quickdraw" to get an idea of how the Mac generally used to work. Most of it is still true today.