This book is a discussion of theatrical improvisation techniques and how they can be applied to roleplaying games.
It's a fairly short book, but densely packed with techniques and tips.
It feels like I will need a bit of time to digest the advice, and probably many months of trying it out before I can tell how useful they are. On the other hand, everything reads like it ought to improve play.
So, provisionally it is very good, and I'll try and post again later with an update on how well the advice helps in play.