Causality: Models, Reasoning, and Inference (Book)
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This one isn't free, but it seems like an excellent read.
http://bayes.cs.ucla.edu/BOOK-2K/
In the Preface to the Second Edition, J. P. writes:
My main audience remain the students: students of statistics who wonder why instructors are reluctant to discuss causality in class; students of epidemiology who wonder why elementary concepts such as confounding are so hard to define mathematically; students of economics and social science who question the meaning of the parameters they estimate; and, naturally, students of artificial intelligence and cognitive science, who write programs and theories for knowledge discovery, causal explanations, and causal speech.