Depends how far you want to take it... There's plenty of links on the web, but if you're really serious (it can be quite addictive writing one), get hold of th

Depends how far you want to take it... There's plenty of links on the web, but if you're really serious (it can be quite addictive writing one), get hold of the "Raytracing/Rendering bible" : Physically Based Rendering by Matt Pharr and Greg Humphreys. There's also a plethora of research freely available with cutting edge techniques to read.