Well, I don't know about reference works, but the two uses I see most often are to indicate either multiplication, or function application.

Example of multiplication: Ten is what percent of forty? Translation: 10 = WP * 40, with solution WP = 25%.

Example of function application: sine of x, written sin(x).

That's admittedly off the top of my head. I suppose, in more advanced mathematics, you also have "of" meaning the application of an operator, like this: the derivative of f(x), or the integral of f(x), etc.