I echo the Colonist book which should be as good and detailed as the MT Word Builders Handbook.
Then a mix of 3I stuff and one generic:
So a History of the Fifth Frontier War, written as just that, a blow by blow history with some stats and adventure seeds, but keep the core game at 1105 so this is there if you want to move into the VFW.
A big fat book of Less Well Known Aliens, the Virushi, the Vegans, Bwaps, Dandelions, all the guys in the GURPS Aliens 4 book, Dolphins, Chimps, and one or twelfty new ones. That could well be a two parter..
Humaniti, again like GURPS, all the little sub species of humans dotted around.
Apocalypse NOW! A book about post apocalypse SF gaming.. which would be useable in the OTU for any of the many Long Nights and planetary collapse, but also allow Mad Max or Planet of the Apes or Zombie Apocalypse games. [It'd make a cool pairing with the Colonist book, build it up with one and tear it down with the other!]
"Down with the Empire", rebels, traitors and revolutionaries in the Third Imperium. Just what do the Ine Givar believe, and who else opposes the aristocratic plutocratic tyranny of the Empire?