I'm not sure what I was expecting when I ordered this book, but I must admit that the clincher (for me, anyway) was the set of example questions listed on the product description - you have to admit, they were funny and intriguing.
Having read the book, I don't think I'm much better at "estimating damn near anything". In the first chapter, the book gives you some advice on how to start addressing a question - any question of approximation - and the rest of the book provides many exercises.
The gist of advice on approximating anything is: take a guess! How many dirt bike racers are in New York? Well, round up the total population of New York, apply exponential notation, decide on a reasonable maximum and minimum possible number of racers, etc.
Of course, there are some questions within the book that require knowledge of specific values, such as the elasticity of rubber, the pull of gravity, and the distance between the Earth and the Moon. For these questions you'll find the values at the back of the book. For other such values in real life just hope you paid attention in school.
There is no secret art in guesstimating anything - either take an educated guess or use some degree of common sense. I was hoping for something new to take away from this book, but found very little.
The sample problems presented in the book are great, though, and it has a very good format.