It is a book written by a brilliant young man in his early twenties, almost a century ago, but is still up to date. As such, it has a refreshing and bold style. His objective is to put the process of reasoning into a scientific framework by organizing different methods with which one can improve his/her thinking efficacy.
He provides a new perspective on concentration by defining it as the process of persistently moving towards a well-defined objective rather than fixating one's thought on a static object. He also provides interesting discussions on debate/conversation, prejudice/uncertainty, reading and writing to help thinking. His list of further reading material in the last chapter of the book is of great value, even a century after it was written, which also indicates the long lasting value of this great piece of work.
This book is my primary travel companion, and it succeeds in provoking new thoughts almost every time I skim through it. Thus, I recommend it to all those who wish to make better use of the "human machine" :)
PS - Later on (53 years after its publication), Hazlitt has written of his thoughts on how to improve upon this book. An outline of these thoughts are presented in "The wisdom of Henry Hazlitt", pp 45-78, which is available online at http://mises.org/books/wisdom.pdf