I noticed in my web traffic that I had some visitors coming from what appears to be a fantastic document on Renderman and Mantra shaders in Houdini over at render.ru. I unfortunately don’t understand the russian language at all, but Google Translate has let me read a bit of it. I’d definitely recommend you take a look at this if you want to get into shader building. I’d send you an email of congratulations, wiTcHer, but the website doesn’t appear to want to let me login (I probably messed something up while trying to register). The Houdini community could really stand to have this available in English!