Apple has revamped the ADC online technical Reference Library.

It looks easy to navigate and find what you're looking for. There's so much more information than what used to be in the original huge tome Inside Macintosh, yet it's probably even easier to use. The documentation, sample code, Q&A, and release notes are gathered together and are well-organized.

I'll probably be using this even more now as I get a chance to update and Carbonize and Aquify my old HexWrench utility.

Another handy reference I click to often is the Single Unix Specification. It's much more basic and covers a much smaller API surface area, so its spare interface works OK, although occasionally you have to know which header file something is owned by in order to find it.