Sigmund Freud's 'Interpretation of Dreams' is a seminal work published in 1900 that lays the foundation for his theories of psychoanalysis, proposing that dreams are a window into the unconscious mind. This book introduced concepts like the distinction between the manifest content (the literal storyline) and the latent content (the hidden psychological meaning) of dreams, emphasizing how these reflections of inner desires can influence human behavior and thoughts.