The official RenderMan Interface 3.1 specification is surprisingly readable for a standard document, but not in any way a tutorial. A must for any serious RenderMan user, it's available directly from Pixar for $15. Their address is:
Pixar
1001 West Cutting Blvd.
Richmond, CA 94804
More accessable is _The RenderMan Companion: A Programmer's Guide
to Realistic Computer Graphics_, by Steve Upstill (Addison-Wesley, 1989, ISBN 0-201-50868-0). _RC_ is an excellent introduction to the procedural API and the Shading Language. Unfortunately, it doesn't address RIB at all.
If you're interested in SL, or procedural shading in general, check out _Textures and Modeling: A Procedural Approach_, by Ebert, Musgrave, Peachey, Perlin, and Worley (Academic Press, 1994, ISBN 0-12-227760-6). The source code (including RM shaders) can be found at URL ftp://archive.cs.umbc.edu/pub/textures>.
Moving to the more obscure, there are good course notes from Siggraph '90 (course 18 - The RenderMan Interface & Shading Language) and Siggraph '92 (course 21 - Writing RenderMan Shaders).
Here are some technical papers that discuss either the RenderMan Interface or some of the technologies behind its implementation(s):
Cook, Carpenter, and Catmull. "The Reyes Image Rendering Architecture", Computer Graphics 21(4):95-102, 1987. (Describes the rendering method that prman uses.)
Hanrahan and Lawson. "A Language for Shading and Lighting Calculations", Computer Graphics 24(4):289-298, August 1990. (Describes SL.)
Reeves, Salesin, and Cook. "Rendering antialiased shadows with depth maps", Computer Graphics 21(4):283-291, July 1987. (Describes the methods that prman uses to compute shadows.)
Slusallek, Pflaum, and Seidel. "Implementing RenderMan--Practice, Problems and Enhancements." Proceedings of Eurographics '94. (Describes their implementation.)
