As an ESOL teacher, I think this is an important read for anyone in my profession.
To argue why I think so, I will use a quick sports analogy: Mixed Martial Arts or MMA.
In MMA, the athletes combine many different combat sports--boxing, jiu jitsu, wrestling, etc. As language teachers, I feel we should also be able to combine many different methods and to choose the best one when appropriate.
For example, some language-learning situations may call for some Audiolingual Method. Some language-learning situations may call for Task-Based Instruction. Some may call for Communicative Language Teaching. Sometimes a combination of multiple methods is appropriate.
This book acts as a great introduction to some of the most popular methods and trends in Language Teaching over the years. It asks the reader to play the "believing game" with each method, and try to find the good in each method, even if we don't agree with everything in said method. I think this is a great approach, as I feel there are things we can learn and take from every method, even ones that are now considered "outdated."
My only criticism is sometimes the book got a bit repetitive and redundant in its breakdown of the principles of each method.
However, overall, it is a must-read for any language teacher. Just as a good MMA fighter must have knowledge of and competence in all the different combat sports, a good ESOL teacher should have knowledge of and competence in the various teaching methods that are available. This book is great training camp for Language Teachers before they step into the octagon, nay, the classroom.