Free books over at Green Tea Press.
All of our books are available under free license that allow readers to copy and distribute the text; they are also free to modify it, which allows them to adapt the book to different needs, and to help develop new material.
They have some great stuff like:
Think Bayes: Bayesian Statistics in Python
Think Python: How To Think Like a Computer Scientist
Think Stats: Probability and Statistics for Programmers
Think Complexity: Exploring Complexity Science with Python