Communicating Sequential Processes (CSP) by Tony Hoare.
From the webpage:
Communicating Sequential Processes, or CSP, is a language for describing patterns of interaction. It is supported by an elegant, mathematical theory, a set of proof tools, and an extensive literature. The book Communicating Sequential Processes was first published in 1985 by Prentice Hall International (who have kindly released the copyright); it is an excellent introduction to the language, and also to the mathematical theory.
An electronic version of the book has been produced, and may be copied, printed, and distributed free of charge. However, such copying, printing, or distribution may not: be carried out for commercial gain; or – for copyright reasons – take place within India, Pakistan, Bangladesh, Sri Lanka, or the Maldives; or involve any modification to the document itself.
Mailing list: csp-announce@comlab.ox.ac.uk
.Enjoy!