Think Complexity is an introduction to complexity science using Python. The premise of this book (and the other books in the Think X series) is that if you know how to program, you can use that skill to learn other things.
The first edition is published by O'Reilly Media. You can also order it from Amazon.
The second edition is a work in progress, available in PDF from Green Tea Press.
The book is under a Creative Commons license: Attribution-NonCommercial-ShareAlike 3.0 Unported. The code is under the GNU GPLv3.
Other Free Books by Allen Downey are available from Green Tea Press.
Tutorial
I offer a half-day tutorial based on material from Think Complexity. Information about the tutorial is here.