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We didn't get rich from sales of the book but it was influential. Many of our ideas ended up in the UML, and the UML's Object Constraint Language was directly based on our work. Also, the book remains probably the most comprehensive reference on the use of state machines to describe object behaviour.
The book has been out of print for some while, but if you would like to take a look the whole book is now available online at www.syntropy.co.uk/syntropy. I'd love to know what you think.