The first and easiest reference is to look in Wikipedia at "The Monte Carlo method for photon transport". Beyond that, I've put a few books on reserve in Evans library, under Math 647:

• "A Monte Carlo Primer," by Dupree and Fraley (QC20.7.M65). This book focuses on neutron transport, but it has the basics.
• "Selected papers on tissue optics: applications in medical diagnostics and therapy," ed. by V. Tuchin (QH642.S45 1994).  Much more than you need to know.
• "Optical-Thermal Response of Laser-Irradiated Tissue," ed. by Welch and van Gemert (QH643.O65 1995). (I'll put this on reserve in the next few days.) Chapter 4 is a nice explanation of the method.
• I stole this project from David Dobson, at University of Utah. Here's the page for his modeling class from 2003; the potato project is the second one, and he provides some more references.