For the $10 price tags, I would recommend any of the alien books, assuming you have interest in enough of the races. Humaniti and Alien Races 4, in particular, are mixed bags, but at $10, they can be quite useful, and they are all very background heavy.
Also, all of the 'reused' races (e.g. Darrians, Bwaps) are completely faithful to their original treatment, but provide a lot of the detail that was missing from that original treatment. For example, the Darrians as shown in Humaniti are the exact same Darrians shown in Alien Module 8. However, the rest of the history is fleshed out (AM8 inexplicably stops at 788 or so), and their current condition is better described.
I will admit that many of the new races are questionable at best, absolute wastes of print in others.
Outside GT, any of the historical books can be exceptionally useful. If you plan on doing something like, say, Firefly, then GURPS Old West can be a tremendous boon.
Also, three of my favorite non-GT GURPS books are: Mars and Alternate Earths 1 & 2.
Mars provides great information on what living on Mars would entail, what terraforming Mars includes, and some options on going 'space opera' with Mars. And while you likely aren't going to find Mars in the Spinward Marches, its blood relatives are in great supply.
The Alternate Earths books are a great read all by themselves. They give reasonably detailed descriptions of (appropriately enough) alternate versions of Earth, given some fundamental change in history. Besides being a fun read (I just love Gernsback), they can be a great inspiration for alternative cultures Travellers can find during their wanderings. Several can be lifted whole onto their own world, or you can just steal themes. Either way, you have about a dozen or so worlds just waiting for your players to visit.
[Edit]
FWIW, I also recommend Nobles and Sword Worlds. Both are of great use outside GT.