Classic Traveller is an amazing scifi role playing game dating back from 1977 and still played today - a 5th version is out. Since today is FreeRPGDay, I guess Marc Miller decided to make this one free.
“This is the original text of the 1981 edition of Classic Traveller based on page image Scans. It includes Books 1-2-3, with errata and corrections inserted (where possible; and additional material in an errata appendix). The PDF has been OCR’ed. Margins are upgraded to 6x9 (from 5-1/2 x 8-1/2) for better margins.”
I didn’t know this still existed. At one point I had the 1981 set and I think I played it once or twice. 
You can get the entire Classic Traveller set - all the classic books from 1977 through the mid 80s - all the supplements, modules, even the board game rules/maps/counters - from FFE for $35. You can request a flash drive or CDRom. https://www.farfuture.net/ - the Classic Traveller CDRom.
The Facsimile book is good for the basics. The flash drive is an amazing resource.
I had the original little black books from 1978 or 79 - I was never able to get anyone to play! It was only in the past month that I picked CT back up and have put together a series of games that people are playing in.