Required Textbook:
Realistic Ray Tracing, Second Edition (PaperBack)
Peter Shirley, R. Keith Morley
At adlibris.com
At Bokus.com
A preview on google books

Other textbooks:

• "Physically Based Rendering: From Theory to Implementation", Matt Pharr and Greg Humphreys

Research Papers :

• "An improved illumination model for shaded display", Turner Whitted, CACM June 1980
• "Distributed Ray Tracing", Rob Cook, Thomas Porter, Loren Carpenter, SIGGRAPH 1984

Other good reference material on the web :

• Global Illumination Compendium
• "Advanced Global Illumination Using Photon Mapping", Wojciech Jarosz, Henrik Wann Jensen, Craig Donner, SIGGRAPH 2008 course notes.
• Textbook Principles of Digital Image Synthesis, by Andrew Glassner