What are the best free resources to learn OS X all the way to the advance level?

[oro79]

What are the best free resources to learn OS X all the way to the advance level?

 

[GGJstudios]

This should get you started: Helpful Information for Any Mac User

 

[tkermit]

At the advanced level, the MAC OS X Reference Library can be quite helpful, possibly even for non-programmers.

Let's say, for example, you wanted to know how the system determines what programs can open which files:

Launch Services Concepts