Just coming here to give my usual thank you/plug to Allen B. Downey for all of his wonderfully written (and free!) books. Aside from The Little Book of Semaphores, here are a few of my favorites:
Thank you for these links. I have read semaphores book and I recommend it to others. Didn't know that there were other good titles from Downey. Planning on reading them all.
How To Think Like A (Functional) Programmer ("Think OCaml") - http://greenteapress.com/thinkocaml/index.html
Think Operating Sytems - http://greenteapress.com/thinkos/index.html
Think Bayes - http://greenteapress.com/wp/think-bayes/
Think Stats - http://greenteapress.com/thinkstats2/index.html
Also important is his section about how to help free books: http://greenteapress.com/easy.html