J. Budziszewski, Written on the Heart: The Case for Natural Law (InterVarsity 1997).
A very readable, entertaining, and basic introduction to natural law theory. This is the perfect resource for the busy law student who is willing to take the time to read it. Despite its accessibility, it is a scholarly primer, with index and numerous notes (228 pages total).
If you would like a shorter, more recent work to just get your feet wet, take a look at Budziszewski's more recent and much shorter book Natural Law for Lawyers (2006). It is also quite readable and perhaps slightly more approachable.
Another great book by Budziszewski, What We Can’t Not Know (2003), is less concerned with instruction on natural law theory and is more helpful in developing an understanding of natural law theory as an evangelistic and apologetics tool (see page 23). For more on this book, check out our blog post "What We Can't Not Know."