The Master Key System is a personal development book by Charles F. Haanel, published in 1916. It is a correspondence course that gives an overview of life and provides a blueprint for achieving success.

It was a popular book at the time, and is still a bestseller today. It has also been translated into many languages and is a source of inspiration for many people.

Why is it so successful? It is a complete and easy-to-understand course, which gives practical advice on how to achieve success. It is based on the idea that we are all part of a great big company, and we can all achieve success if we work together.