This collection was first published in 1938 and contains English translations (by one of Freud's first American followers, A.A. Brill, with an introduction by Brill) of six books by Sigmund Freud:
Psychopathology of Everyday Life (1904)
The Interpretation of Dreams (1899)
Wit and Its Relation to the Unconscious (1905)
Three Contributions to the Theory of Sex (1905)
Totem and Taboo (1913)
The History of the Psychoanalytic Movement (1914)
Freud is the inventor (or co-inventor with Josef Breuer, in some tellings) of psychoanalysis, a revolutionary new treatment for mental illness. Through the practice of psychoanalysis on a large number of patients, Freud not only alleviated a great deal of suffering, but also developed a theory of the mind, which he believed applied to all human beings, not just those afflicted with mental illness. He also believed that one could use his theories to gain insights into other areas of study, including history, anthropology, sociology, religion, and the arts. Although many of Freud's specific findings have been challenged or rejected, often because they have not been proven by modern scientific research methods, and even though formal psychoanalysis is rarely practiced anymore, many of the basic principles of Freud's theory and practice form the foundation of current psychology and psychotherapy; Freudian concepts also permeate the broader culture and the layperson's understanding of how the mind works.
The six works in this collection contain many of Freud's most notable ideas and discoveries, which are presented in the context of the specific topic being discussed. As far as I know, Freud never wrote a grand summary of his theories in one volume. But by reading all six books, one becomes familiar with the free association method of psychoanalysis, the three aspects of mind (conscious, preconscious, and unconscious), the idea that sexuality is a force in humans from birth, the Oedipal conflict, penis envy, fear of castration, and the notion that dreams are the disguised fulfillment of unconscious (infantile) wishes. We learn that slips of the tongue, lapses of memory, and humor may be messages from our unconscious mind and that certain aspects of culture and religion may be based on primitive or childhood sexual conflicts. Two major elements of Freud's thinking that are missing from these books are the theory of the ego, id, and superego, which barely gets mentioned, and the idea of the conflict between the life force (sexuality) and death force.
I don't know why they put Psychopathology of Everyday Life first, out of chronological order. The Interpretation of Dreams comes first chronologically and is considered Freud's most important book, with Three Contributions to the Theory of Sex (also known as Three Essays on the Theory of Sexuality) as the next most important.
Another note for purists is that the version of The Interpretation of Dreams is abridged somewhat by the editor, who cut much of the section in which Freud reviews previous theories on dream interpretation. Also, all the books here include revisions and updates, often in footnotes, added by Freud (and Brill) after the initial publication of the book. This is particularly apparent in this edition of The Interpretation of Dreams, which contains many updates (some of which contradict the original 1899 text).
In order to place Freud's thought in a modern context, I plan to read a book or books that include the various challenges to Freudian theory and practice that have arisen in the century since these books were written.