You can inspect a few things in the .hi files:
    ghc --show-iface abc.hi
and it is usually used to see what conclusions ghc was able to reach about your code (since your code will also generate .hi files). I don't think it's much use looking into library .hi files however unless you are yourself the library author. The library documentation is usually the best way to learn about the library.

Regards,
Abhay