"The Soul's Code" is a book by James Hillman that explores the idea that each person has a unique calling or purpose in life, which he refers to as the "acorn theory." Hillman suggests that just as an acorn contains the potential to become an oak tree, individuals possess an inherent essence that guides their development and choices. The book emphasizes the importance of understanding this inner calling to lead a fulfilling life. Hillman argues that societal expectations often overshadow personal desires, and reconnecting with one's true self is essential for personal growth and authenticity.