It consisted of three little black books:
1. Characters and Combat
3. Worlds and Adventures
That’s it and it was all you needed to get started. Define your character, how to get to other planets, and what you find once you get there.
Oh there was more, GDW published additional rule books, adventures, and other supplements, including two reworkings of the rule set. Those were MegaTraveller and Traveller: The New Era.
Steve Jackson Games put out a licensed version for their GURPS rules, and Mongoose Publishing is producing books with that LBB (Little Black Book) feel.
I’ve enjoyed Traveller for a lot of years, and it has a very rich and detailed game history that you can use or ignore as you desire.