The definitive documentation for programming the Mac, Inside Macintosh is a series of books. How many? I'm not sure. They take up 4 or 5 feet of bookshelf space, that's for sure. But they are the reference for Mac developers. I think Apple mostly distributes them on CD-ROM now.

They cover all the major aspects of coding for the Mac: things like the Toolbox, Quicktime, Sound Manager and a lot of other stuff. I'm sure none of it applies to Mac OS X, but if you want to know how that cute little box worked back in the day, the Inside Mac books are the beginning and the end of pre-OS X API documentation.