Learn with a purpose. I think you'll learn best if you have in mind some goal -- fix this or add that functionality. Start looking for the likely places where you will need to make your changes. Follow threads of control backwards to find out how to get to that point in the code. Also, take a "bird's eye" view of the code -- look at the layout and structure. Good code will have descriptive names that tells you want each class and method is for. See if you can recognize implementation patterns and see where/how they are used. Don't put too much stock in the documentation -- documents can be excellent, but they are often out of sync with what the code really does. Let the code itself be the best documentation.
