The Little Book of Semaphores
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.htmlThink Bayes : http://www.greenteapress.com/thinkbayes/index.htmlThink Complexity : http://www.greenteapress.com/complexity/index.htmlThink Stats : http://www.greenteapress.com/thinkstats/index.html