What is the proper way to add outside source files to a custom plugin?

If I have a library source consisting of *.h and *.cpp files located outside the unreal4 folder , What is the proper way of adding them to a custom plugin so I can access them from inside the UE4 solution?

I have tried adding them directly to the my plugin’s folder but this doesn’t work right. It breaks the include pathing.

there is a wiki entry about adding static libraries.
If you follow this tutorial exactly , you will be able to add your own custom library.
I tried it and it works really well.
Add Custom Library to UE4 C++ project