The author Alan B. Downey has released quite a few free books:

Check out http://www.greenteapress.com/ including:

Think Python : http://www.greenteapress.com/thinkpython/thinkpython.html

Think Bayes : http://www.greenteapress.com/thinkbayes/index.html

Think Complexity : http://www.greenteapress.com/complexity/index.html

Think Stats : http://www.greenteapress.com/thinkstats/index.html