Real Time Ray Tracing
October 2001 - Thomas Ludwig
This article gives ideas to various optimizations and cheats that can be employed to do real-time ray tracing. It doesn't give any detailed explanation of the optimizations but the article is interesting anyway.
Raytracing Topics & Techniques
Sep, 2004 - Jacco Bikker
Very good series on ray tracing. Included topics are kd-trees, ray-triangle intersection, reflection/refraction, textures, bumpmaps, etc.
Approximate Ray-Tracing on the GPU with Distance Impostors
László Szirmay-Kalos, Barnabás Aszódi, István Lazányi, Mátyás Premecz
The result looks good and may be worth investigating if these effects are wanted.