I'm sure you will need the Companion
, as it gives you much alternate rules you may like.
The Central Supply Catalogue
is also a must, as it gives you plenty of stuff your players will love.
After those 2, it really depends of your needs. I would recommand a setting book to have a playground to explore :
- Behind the Claw
gives you 2 sectors with much informations, at the frontiers, where much can happen.
- The Sword Worlds
gives you a sector with a distinct "viking" vibe you may well enjoy.
- The Third Imperium
gives you much information about the nobility, navy, history of the dominant human power. Plenty of opportunities in those civilised worlds
Or you can opt for one campaign :
- The Pirates of Drinax
: 3 books, one for the Trojan Reach (just aside the sectors explained in Behind the Claw), one for ships and one big adventure. It needs proactive players but it's one of the best campaign made. You will have also several other published adventures you can set inside.
- The Great Rift
: Dwelve in the unknown in this setting, but i'm not the right people to talk about it.