Book 0: An Introduction To Traveller, Game Designers Workshop, 1981. I think this is a booklet that came in the Deluxe Set with the three main booklets and a hex map or something.
Traveller is a science-fiction role-playing game set in the distant future, when humanity has made the leap to the stars and interstellar travel is as common as international travel is today. This means that Traveller is set against a background drawn from adventure-oriented science fiction literature, and the scope and breadth of the game are limited only by the imagination and skill of the players and their referee. Players are no longer limited to wandering inside a single underground labyrinth, to exploring a single continent, or even a single world. In Traveller there is an entire universe to be explored. Almost any situation which occurs in any SF novel, movie, or short story can be recreated in Traveller with a little work on the part of the referee.
In Traveller, mankind has conquered the stars, and travels from one stellar system to another as easily as present day Terrans can travel from one continent to another. The tremendous distances involved, however, dictate that interstellar voyages can take weeks, months, and sometimes even years. A situation similar to earth in the eighteenth century is created, where communication is limited to the speed of travel, and the stage is set for adventure in a grand fashion, with all the trappings of the classic space opera: giant, star-spanning empires (good, evil, or both), huge starfleets, wily interstellar merchants (or pirates, depending upon your point of view), complex diplomatic maneuvers, larger than life heroes, heroines, and villains — the mind boggles.