Little Black Books 2 and 3

Traveller - a person who journeys through space
Book 1 covers Characters and Combat and it can be used without the other books if the referee is interested in exploring a story that will develop through play on a single planet. Honestly, that is how I like to run Traveller. The Traveller system, even in its earliest edition is much more than characters and combat. The original Black Box comes with three little black books. Book 2 is titled Starships and introduces rules for travelling through space (hence the Traveller name). Book 3 Worlds and Adventures includes information on worlds, equipment and psionics.

Book 2 Starships provides information on the standard hull interplanetary and jump capable space vessels. Traveller uses a bit of math (square root) in calculating how long it takes to get somewhere using maneuver drive propulsion. Jump drive travel is between star systems and takes about a week. Book 2 also covers ship design and weaponry and ship combat. Traveller uses a vector system for maneuvering spaceships, which means it takes momentum and gravity into account. Yes, Traveller relies a bit more on science and physics than some space opera style games. The math is minimal and it is quite simplified (space combat is played out on a two dimensional surface). There is just enough there to give the impression one is dealing with the game in a realistic, scientific manner.
Economics, trade and commerce can feature as a big part of a Traveller game if that is the direction the players want to take. Financing a ship, haggling for cargo prices and turning a profit transporting goods, passengers and mail can be one way to play the game - sometimes referred to as "space trucking". Traveller is intended to cover several play styles. It supports solo and group play, with or without a referee. This has been very formative in my own style of gaming which at times includes solo and unsupervised group play.

Book 3 Worlds and Adventures provides tools for exploring space and encountering new life forms and civilizations. Book 3 also includes the Traveller equipment list. Traveller exists in a far future as imagined in the late 1970's. One aspect which makes the Traveller universe so playable is the absence of faster than light communication. Messages travel at the speed of the jump drive. In other words, players are isolated and must often deal with trouble on their own.
Black Book Traveller is humanocentric. Player characters are humans. Robots exist, but they serve humans. The magic of Traveller comes in the form of psionics, ESP, telekinesis, and so on. This is the default as covered in the original system material, but its space...anything is possible and the referee and players can introduce anything imaginable. Book 3 ends with a strong suggestion for those playing Traveller to draw upon science fiction literature for game inspiration.
The Traveller subtitle, Science-Fiction Adventure in the Far Future, suggests a wide array of possibilities. Space exploration, colonization, conflict, trading, piracy, diplomacy, espionage and more, are all suitable for Traveller gameplay. My early days in the hobby were spent pulling stories from sources outside gaming, from novels, comics, movies and artwork, and converting them all into game material. (The 1979 film Alien served as inspiration for one such adventure.) Traveller is a vehicle for most any adventure story one might want to tell in a far future (or past). The Little Black Books, much like the Little Brown Books, comprise a referee's toolkit. The possibilities are endless, but Traveller does require the referee to do some design work (unless you are content running one of the published modules).