The Book: 21 Day Habit Developing healthy habits is one of the keys to successful living. Experts say that if we do something for 21 days straight, we can turn it into a habit for a lifetime. The Book: 21 Day Habit is designed to get you started. Set aside just a few minutes a day to read The Book: 21 Day Habit, and when you are finished, move right into The Book and keep on going!