Short version: Almost any of them can be useful, regardless of era, depending upon what sorts of things you and your players tend to do in the campaign. Here's a breif rundown on the books I'm familiar with.
Starships of the Galaxy: Good if you're going to have a lot of space combat. The ship maneuver rules are really useful if characters will spend a lot of screen time flying their ships. If you're going to go there, this book is almost essential.
Threats of the Galaxy: A lot of stat blocks (that don't follow the rules) organized in a poor fashion (
WotC never repeated that book-layout style) and with tons of "hidden" sidebars containing mechanics of questionable game balance. I really can't recommend this one, unless you just need to have reems of stat blocks for things to kill.
Knights of the Old Republic: Tons of equipment and cultural information for the major groups of the era. This includes Mandalorians, Sith, and the Republic military. Useful for the organizations and equipment, as well as some of the generic foes. Again, there's some difficulty getting the designers to follow their own rules (but not much).
The Force Unleashed: Filled with some of the most over-the-top force mechanics in Saga, this book also has a lot of great information about the organization and structure of the Empire and associated criminal groups. Great background and fluff, but the utility depends upon how prominent the Empire or organized crime will be.
Scum and Villainy: A
lot of new equipment mechanics, as well as a host of non-Force abilities for characters. Some great background on criminal and bounty hunter groups as well. Probably one of the most generally useful books, unless you campaigns center around the Force.
I hope that helped some. Good luck.