"The Interpretation of Dreams" is a book by Sigmund Freud, the first edition of which came out in 1900.
At the beginning of Chapter One, Freud describes his work thus:
In the following pages, I shall demonstrate that there is a psychological technique which makes it possible to interpret dreams, and that on the application of this technique, every dream will reveal itself as a psychological structure, full of significance, and one which may be assigned to a specific place in the psychic activities of the waking state. Further, I shall endeavour to elucidate the processes which underlie the strangeness and obscurity of dreams, and to deduce from these processes the nature of the psychic forces whose conflict or co-operation is responsible for our dreams.
The book introduces the Id, the Ego, and the Superego, and describes Freud's theory of the unconscious with respect to Dream interpretation. Widely considered to be his most important contribution to Psychology, Freud said of this work, "Insight such as this falls to one's lot but once in a lifetime."
The early editions of the book are in the public domain, and available in electronic form over the internet.