The UCSD Prof. sheds a lot of light into the absolute basics. A very good "first" book for anyone interested in Realistic Image Synthesis using Global Illumination. It's not just about photon mapping; Rather the initial chapters give a very good overview (and drawbacks) of other approaches of solving GI.
I was very much interested in learning the nitty-gritty details, particularly for rendering (realistic) caustic effects, and enjoyed the whole book and the umpteen number of high dimensional integrals and derivations of the same giving a good intuition for the Monte Carlo Integration.
All in all, I highly recommend this book!