Impro: Improvisation and the Theatre (1979) by Keith Johnstone is a highly recommended book on improvisation by Keith Johnstone who is widely credited with inventing modern improvisational theatre. The book is a classic text.
The book first has biographical information about Johnstone and he goes into how he found school to be constraining in the way it forces children to suppress a lot of creativity. Johnstone’s views here are interesting and set the scene for what is to come.
Then Status and status games are discussed. Here Johnstone gets into the details of improvisational theatre and suggest games and techniques to try. The next chapters on Spontaneity, narrative skill and masks and trance all go further into this. The mask chapter makes interesting remarks about how masks and trance in other societies are used.
The book is fairly short and well worth reading but it is not a great read. Anyone who is doing impro should get a lot out of it however.