This seminar by Mark Passio is without question the best one on Natural Law that I have seen in the last decade.
Natural Law can be defined as Universal, non man-made, binding and immutable conditions that govern the consequenses of behavior. It's is a body of Universal Spiritual Laws which act as the governing dynamics of Consciousness and is also known as Karmic Law. This is the working definition given in this presentation.
To put it simply and in more common vernacular Natural Law means whatever you do comes back to you or you shall reap what you sow. Unlike man-made or religious laws, Natural, i.e. Universal Laws apply whether you believe in them or not.
In his presentation Mark Passio refers to a book written by Jeremy Locke called The End of All Evil. It's out of print and second hand editions sold on Amazon currently cost between USD 1.200 and USD 10.000 but you can read the pdf version online. The link is provided below.