Many of the books at Green Tea Press[1] are available for free:
- Think Python: How To Think Like a Computer Scientist
- Think Bayes: Bayesian Statistics in Python
- Think Complexity: Exploring Complexity Science with Python
- Think Stats: Probability and Statistics for Programmers
- The Little Book of Semaphores
- Physical Modeling in MATLAB
- Learning Perl the Hard Way
few others. Do check the site.
Secondly, books by Al Sweigart[2] are also freely available. They include:
- Automate the Boring Stuff with Python[3]
- Hacking Secret Ciphers with Python
- Invent Your Own Computer Games with Python
- Making Games with Python & Pygame
[0] - https://github.com/vhf/free-programming-books
[1] - http://www.greenteapress.com/