There are two current paperback versions of Psycho-Cybernetics. Both are rather ugly reminding you of high school science textbooks. The Wilshire edition has an image of a man's torso inside a triangle, with a laser beam running through his head. What tips you off that it is in fact a self-help book is a yellow badge saying 'Over 5,000,000 copies sold'.

This figure, though, is only part of the story. The non-profit Psycho-Cybernetics Foundation estimates that worldwide sales of the book, including the editions of five US publishers and the many foreign translations, from 1960-1997, exceeds 25 million copies.

The word cybernetic comes from the Greek for 'steersman', and in the modern sense usually refers to systems of control and communication in machines and animals: how, for instance, a computer or a mouse organises itself to achieve a task. Maltz applied the science to man to form psycho-cybernetics. However, while inspired by the development of sophisticated machines, his book denounced the idea that man can be reduced to a machine. Psycho-cybernetics bridges the gap between our mechanistic models of the brain's functioning (cliches like 'Your brain is a wonderful computer'), and the knowledge of ourselves as being a lot more than machine.

The founder of cybernetics was American mathematician Norbert Wiener, who spent World War Two refining guided missile technology. Maltz thought: why could the technology behind guided missiles of a constant feedback loop in order to maintain direction not be applied to human achievement? He realised the key point about the loop is that it gains an automaticity when the target or goal is very clearly fixed. Cybernetics appeared such a breakthrough to Maltz because its implication was that achievement was a matter of choice. Most important to the dynamic of achieving was the 'what' (the target), rather than the 'how' (the path).

Maltz was a plastic surgeon. His life centred around giving people a good image of themselves in the mirror. Distinguished as he was in the field, he was at a loss to explain why a minority of patients were no happier after operation than before, even if disfiguring scars or other malformations had been removed. He found himself drawn into the new self-image psychology, which held that we generally conform in action and thought to a deep image of ourselves. Without a change to this inner image, patients would still feel themselves to be ugly, however excellent the cosmetic work.

Maltz came to believe that self-image was the 'golden key' to a better life. Without an understanding of it, we might forever be fiddling around the 'circumference of the self' - instead of its centre. Positive thinking, for instance, could be of no use if it simply related to particular external circumstances. Saying 'I will get this job' will not do anything if the idea of being in the job is not consistent with how you see yourself deep down.

As a plastic surgeon who gave people new faces, Maltz also became interested in self-image psychology. This suggested that we acquire our self-image through our beliefs about ourselves, which grow out of past experience of success and failure and how others see us. Maltz argued that both are unworthy of the privilege of determining our basic psychological template. The crucial and fascinating point about the self-image is that it is value neutral, that is, it doesn't care if is empowering or destructive, but will form itself simply according to what psychological food it is fed. Maltz realised that winning images of the self could replace negative ones, denying any authority to past events. The beauty of self-image was that while it was the supreme factor in determining success or failure, it was also extremely malleable.

A lot of self-help writing is about goals, but how does goal-setting work? Why does it work? Maltz was the first to explore the actual machinery of it, and in doing this he has been a key influence to a generation of success writers. The emphasis on positive self-image paved the way for hundreds of books on the power of affirmation and visualisation techniques. It has sold in its millions because it provides a scientific rationale for dream fulfilment.

Notwithstanding its 'Reader's Digest' style of writing, Psycho-Cybernetics is, in fact, a textbook. The science and computing references are now outdated, but the principles of cybernetics have only grown in influence. Complexity theory, artificial intelligence and cognitive science all grew out of the cybernetic understanding of how the non-physical, the 'ghost in the machine', guides matter. This makes Psycho-Cybernetics the perfect self-help book for a technological culture.