Please note that this review is sorted from bad to good, with a problematic caveat at the end, though.
The biggest problem I see in the document is the repetition, though. I got the distinct impression that about half the essays suggest using a seemingly standard approach from improvised acting (as in theatre acting). Sure, each contributor takes a slightly different approach to it, and some are easier to understand than others, thus making the variants interesting, but in the end, it is basically the same advice clothed in slightly different apparel.
If you take this into account, the 105 pages of text become maybe some 50 to 60 pages of effective text. Still quite nice, but it may appear a bit pricey.
In addition, each individual essay is rather short. At times, you get the feeling you would want to read a bit more on the suggested approach or get more details. Mind you, the essays get the message across, but I personally would have liked to have some of them longer (maybe double their current size).
There is indeed some very solid advice there and a few approaches (despite the repetition) to try out for GMs. As such, I think the book can deliver for GMs who want to switch from rail-roaded role-playing to more improvised approaches or those who feel overwhelmed by the surprises that come up during play. While there are essays dealing with the planning of the session, the majority focuses on actually handling the situation (and even those dealing with the planning turn towards the actual play, of course).
As a bonus, the essays in this text are all really well-written and an entertaining read. Anecdotes and examples are often amusing and drive the points described home.
I also want to mention that I didn't notice any typos or grammar mistakes (save maybe a single time where an unmodified "weird" is seemingly used as a comparative, albeit that may very well be a deliberate decision of the author rather than a mistake). While this may seem a minor point, I personally find it rather distracting if an item is full of typos or mistakes (like using "dice" for the singular of our favorite randomizer!). I really want to send some praise to the authors and the proof readers and everyone else who helped keep the quality that high. It is really appreciated.
Now, for the special caveat, I need to point out and explain an easily over-looked aspect. Besides your standard, multiplayer RPGs, there are also solo or solitaire RPGs, probably the most successful engine being Mythic's Game Master Emulator. Those RPGs allow gamers to enjoy RPGs all by themselves without the need for other players. This becomes possible, at least with Mythic's engine, by keeping the answers of the GM Emulator very vague and requiring the player to interpret the controlled randomly generated and abstract results. As a consequence, solitaire RPGs played using this or a similar engine is highly improvised gaming where the player is partly GM (providing details) but mainly player. Therefore, a game about improvisation for GMs sounds like a good tool for anyone wishing to play solitaire RPGs. Unfortunately, only very few of the essays center on the GM by themselves. Instead, improvisation is mostly defined by a style of interaction between GM and players. Therefore, while there is some good advice even for solitaire gamers, it is rather little, so I am not sure whether it is worth it for exclusively solitaire players.