The starter book gives you all the basic rules. You can play a few Narn vs Centauri battles with that.
The full rules are in the revised boxed set, along with counters for all the ships. It includes fleet lists for all the main races.
The big expansion is Sky Full of Stars which includes loads of new ships, splits up the league into separate races, boarding actions, advanced fighter rules, planetary assaults, spacestations etc.
The latest expansion is Armageddon with a handful of new big ships and a few rules changes and clarifications.
