You can listen to a great introductory lecture on natural law...

... by Dr. J. Budziszewski on the Intercollegiate Studies Institute website; Budziszewski's hour-long talk, "An Introduction to Classical Natural Law", was given on June 21st at the Seventh Annual Summer Institute at Princeton University. (The ISI site has several hundred lectures available for online listening, on a wide range of philosophical, political, and cultural topics.)

Dr. Budziszewski is the author of several books, including What We Can't Not Know: A Guide (revised edition) published a few months ago by Ignatius Press. Here is an excerpt from that book:

 

And here is an interview with Budziszewski about his book and natural law: