For the standard libraries you can always go to : http://arduino.cc/en/Reference/Libraries
Als for the libraries developed by others, it is dependent how willing they were to make the documentation. And not everyone wants to do that. So you will have to figure it out yourself.
The code for the libaries themselves can always be read as they are supplied in source code. Most libararies are no more than a class put in a .h and a .cpp file. No assembly knowledge required (or at least for most of them).