I bought this because it describes succinctly a lot of games for drama workshops, putting them in context with their purpose. The rationale is important: these games are not mere "warm-up" exercises, but integral to the creation of theatre. The book makes reference to Boal's work and "Theatre Games" (Clive Barker), but the organisation of the work means you'll find what you want more easily here than in Barker's book.
The games are only about one third of the book. It's aimed at the leaders of community theatre projects, including those who work in prisons, youth clubs, with the disabled, and in various other institutions. Johnston provides much useful guidance about workshop dynamics. I especially liked the section the "six polarities" (fixed vs. free, centre vs. edge). This deals with opposites types of behaviour, and the power we have to balance opposites and to choose to act differently.
The facilitation section in the middle of the book flags a little. There is not enough one can say categorically when the core message is "be adaptable". Nevertheless, it's a good resource for anyone who leads drama in small groups, or who undertakes a community project. Know what to do, and know why you do it.