Like a previous reviewer, I was required to read this book for an English class, and also like that reviewer I re-discovered it a few years back.
Senator Hayakawa's main point is that, since it is language we humans use in order to think, and since language has such an extraordinary power to influence others and ourselves, we should pay heed to how we use it and how we interpret it.
In *Language in Thought and Action* Hayakawa discusses ways of better understanding language, and therefore thought, and therefore action, including the use of the "operational definition" and the need to recognize different levels of abstraction.
His essay comparing poetry and advertising is, all by itself, worth the price of the book. You'll never again be so smug about your pronouncements or those of others after reading about two-valued logic versus the multi-valued orientation. You'll learn why the words "Tell me more" can make a difference to you.
I like this book so much I decided to mail several copies to people I know as surprise gifts, along with a letter explaining my enthusiasm for it.
If you use language, if you think, if you act, you should read this book.
Update of November 2009: I am now reading this book for a fourth time. You can read a long excerpt at [...].