I suggest that you follow Amazon's recommendation and buy it with the acompanying sequel "Impro for Storytellers"
Once upon a time there was a brilliant little boy who lived in an alternative universe so he had mercifully never heard of Viola Spolin.
Therefore instead of following standard improv exercises such as 'mirror' or 'tug of war on the imaginary rope' He began creating his own.
These wre not dependant on the Spolin credo of agreeing on the 'Who, What, When and Where'. Rather our young hero was more fascinated by 'Why?' or in plainer English, 'Why should either the audience or the actors give a (expletive deleted) about this improv exercise? What's its value and moreover, could it apply to acting in general?
Consequently through experimentation, a contrary kind of courage ('My acting teachers told me never to make faces as it was untruthful, so I mugged whenever I could') and a curious interest in the transformative power of Mask work--which was out of fashion in The West since the fall of Athens--arrived at some startingly new discoveries for character work as well as Improv.
He then moved to Canada, opened his own troupe, and the rest, as they say is history . . .
Let's see, did I leave anything out? Oh Yes, a practical piece of advice and a cryptic remark to end this review:
1. If the library is burning and you have a choice of saving Stanislavski's trilogy or these two books, save Johnstone's
2. Beware of Boris (to say nothing of Igor)