Since its creation as a new field of science, Systems thinking has influenced most other areas, including the fields I'm active in myself, such as psychology and social sciences.
This out-of-print book is considered so important by the people of the Principia Cybernetica Project that they want this book to reach as an wide audience as possible. As their website indicates: "W. Ross Ashby is one of the founding fathers of both cybernetics and systems theory. He developed such fundamental ideas as the homeostat, the law of requisite variety, the principle of self-organization, and the principle of regulatory models."
Of course, nothing goes above getting your hand on a *real* copy of this book, but the next best solution is getting the free e-book version which the Principia Cybernetica Project published on their web site with the agreement of the Ashby estate (the copyright holders). Just search for Principia Cybernetica on the web, and surf from there.