"As a Man Thinketh" by James Allen is a self-help book that explores the power of thought and its influence on an individual's life. The book's main premise is that thoughts shape an individual's character, circumstances, health, and ultimately, their destiny. Allen emphasizes the importance of self-control and the cultivation of positive thoughts as the keys to personal growth, happiness, and success. He argues that by consciously choosing and nurturing positive thoughts, individuals can overcome adversity, achieve their goals, and create a harmonious and fulfilling life.