The Little Book of Semaphores [pdf]
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:How To Think Like A (Functional) Programmer ("Think OCaml") - http://greenteapress.com/thinkocaml/index.htmlThink Operating Sytems - http://greenteapress.com/thinkos/index.htmlThink Bayes - http://greenteapress.com/wp/think-bayes/Think Stats - http://greenteapress.com/thinkstats2/index.htmlAlso important is his section about how to help free books: http://greenteapress.com/easy.html