Owning almost everything Starwars D20 and running a game since it came out I can tell which books I use the most.
Core Book --Nuff Said
Starships of the Galaxy -- Well I am running a game with smugglers and they like to upgrade thier ships so it is very handy.
Arms and Equipment Guid -- Always use this. Rather than having a Heavy blaster your bad guy has a Merr-Sonn Intimidator. Some of these guns have special little things added to them. Also has a bunch of different scanners, survival gear. droids, armor and vehicles. VERY VERY USEFUL (imho)
Heroes Guide -- New feats, PrC, equipment, cybernetics and FACTIONS.
Power of the Jedi -- Well running a Knights of the Old Rebuplic game with jedi this was important. Also a new player is making a jedi and found it very useful.
Most of my other books don't get used very much. Thats my recomendation