Great books you read in 2009?
herdrick: Judea Pearl's Probabilistic Reasoning in Intelligent Systems: Networks of Plausible Inference.  I'm not finished with it yet, though.  (It might be superceded by his Causality which I'd also like to read.)  It's provided some "aha", scales falling from eyes moments.  I'd guess it's the most important book - for me - I've read in years, perhaps ever.  Highly recommended (with the caveat about his other book perhaps being better).