I'm usually not for people pushing books on HN like this, but this is entirely too relevant to my interests not to thank you for posting this. Visualizing my processes has by far been the hardest part of getting my CS degree, and I have a professor this semester as well as next that is obsessed with machine learning.
So. I can't buy this free book because I'm in the UK?
What?
Is this an ad? News? What are we supposed to do with this submission? How did this make it to the frontpage?
Very high level, but I think http://www.r2d3.us/ is actually a pretty good visual intro, although it skips over information gain/CART metrics.