You've got the three main books, but I recommend the
Arms & Equipment Guide as your next purchase. It's useful for both GMs and Players. It's also $10 cheaper than the hardcovers.
You don't really need
Ultimate Alien Anthology or
Ultimate Adversaries. The players will never know the difference between an official version from UAA of a Chiss or Ithorian vs. you just using a generic human NPC and calling it an alien. They don't get to look at the stats.

If your players want to play something other than the races found in the core rules, then UAA becomes more important to your game. The problem for the GM with this book is it doesn't cover any critters***, just playable sentients. Critters are included in the 1e Alien Anthology (tweak yourself to RCR). Critters, at least some (like the ysalamiri and hoojib), may be found in
Ultimate Adversaries, however...
Ultimate Adversaries is not a book I find particularly useful as GM, but I've never found NPC/villain collections useful regardless of game system. I prefer to make my own. I am irked, however, that UA has a couple things I wanted stats for, such as the Hailfire Droid.
The Rebellion Era Sourcebook was decent, but no great shakes. It's for 1e and OOP, but still cheap. Has a few cool (but way too tiny) maps for Imperial Prisons, Imperial Supply Depots, Asteriod Bases, Echo Base on Hoth, and (yes!) even a Star Destroyer bridge. I prefer the
Knights of the Old Republic era so I don't have to worry about tripping over the toes of Uncle George and the characters/plots from the films (plus, there are thousands of Sith vs. thousands of Jedi!). There is a free 118 page 8MB color fan-made sourcebook by Ghenghis Ska you can get for the KOTOR era. Check the Star Wars RPG forums for download link.
The New Jedi Order Sourcebook: The era didn't interest me so I never looked at it and can't speak for it. If you like the Yuuzahn Vong, then this book becomes more important, otherwise, you can skip it.
If you're a GM, then
Dark Side Sourcebook is very good (but again, for 1e not RCR and is OOP, though easier/cheaper to find than PotJ). WoTC's site has the RCR revisions to DSSB as a free 15 page download (changes are to PrCs, skills, feats; rest of book is unchanged by RCR).
Power of the Jedi is decent, but not as good as DSSB. It is also very rare, OOP, and usually sells for a minimum of $60-80 used on eBay.
The world guides,
Coruscant and the Core Worlds and
Geonosis and the Outer Rim Worlds, are both decent, but not that amazing. I found them more useful for their maps. Most of the planets just did not interest me, and the books didn't cover the planets I wanted to know more about.
Starships of the Galaxy is OOP and pricey on eBay too. It is for 1e not RCR. I haven't given it much of a look because my group dislikes starship combat (not in general, just the way it is handled in SW).
I liked what I've read so far of the super adventure,
Tempest Feud. And I normally
hate super adventures for D&D, so this was a pleasant surprise. It is for 1e, but again, WoTC has the NPC stats converted to the RCR (you have to go to the Star Wars RPG forum "Mess Hall" and you'll find it as a "sticky" toward the top). It's got a map of a ship like Princess Leia's
Tantive IV (better known as the "Rebel Blockade Runner" from Ep. IV).
I'm not sure what your needs are, so I hope this advice helps. Ultimately, you may want to end up owning nearly ALL the books (at least the ones with era-neutral applications depending on what era you want to game in). Each has something usefuyl in it, and while of a generally much higher quality than the D&D line, are still hit or miss. Hey, the galaxy's a big place, so it's even harder to write something that meets everybody's need.
*** Note: You can instantly convert any non-magical/non-psionic D&D critter to SW simply by changing AC to Defense (same number), HP to Vitality Points, and CON to Wound Points.