Aside from the corebook (be advised, there is a 2nd printing with the October 2008 errata incorporated into it), the only book I would say truly qualifies as being essential is Starships of the Galaxy.
Scum & Villainy is helpful if you plan on running a Fringe style game in the vein of Firefly/Serenity, not to mention it has the equipment upgrade rules if your players are interested in that sort of thing.
The Era campaign guides also have some neat stuff, but they are far from essential, especially if you're not playing in that particular era. Yeah, Legacy Era and KOTOR guides are pretty sweet, but neither of them is of much use if your game starts a couple years after the Battle of Endor.
As for Threats of the Galaxy, sometime in the next week or so (I hope), the boys at d20 Radio should have gotten the latest version of my massive NPC file (over 150 NPCs as well as some new equipment) set up as a download. While version 1.2 does draw from a lot of the different sourcebooks (earlier versions pretty much stuck with the corebook), they can easily be tweaked to suit your needs as a GM. I've also tried to update and correct as many of the prior stat block errors as possible; it may not be 100% accurate, but it shouldn't be too far off either (the biggest complaint about TotG is the sheer volume of errors in every single stat block). And best of all, it's free
Once I get word from GM Chris or GM Dave that it's been uploaded, I'll update my sig with the link.