I already have the Core-Book, High Guard, Central Supply Catalogue and the Aslan book in print, as well as the new Vehicle Books (and if I recall correctly the Solomani book) in PDF. Which other books are worth my money?
I'm kinda down on MongTrav but will rise to the occasion because there are some gems. I've starred the ones I would hate to run a game without but the books you do have (HG and CSC) would also be on this list. And frankly, the ones you do have are definitely enough.
*MERCENARY: Everyone hates it. I love it. I've said it before... why is everyone complaining because the artillery ranges are unrealistic? Artillery? Really? The careers are really fun with great differences between them. The expanded event tables for Army/Marines is solid. The Mercenary unit rules... can't speak to them but they do lay out the basics of mercenary contracts etc. Looks good, no experience. There are other reasons people don't like it but I think the careers alone are worth it.
*SCOUNDREL: Yeah, you want this one. Lots of new mechanics, heists, space piracy random charts... great careers. If your campaign will have none of this you can pass but I think it's a gem if you're thinking of dipping into crime.
BELTSTRIKE: Amazingly good. It's a very niche product that focuses only on mining but achieves its goals in spades. The long-term campaign in the back of the book looks like one of the most interesting plotlines I've seen. It's really mainly of use if you're planning on mining and refining in your game. But it's a great read with lots of information. (Combine this with Merchant Prince... hmm)
DILETTANTE: Honestly, I really enjoyed this book. I was pleasantly surprised. This book is another one that is very niche in its approach (see Merchant Prince). I haven't played with the mechanical aspects too much but they looked sound. The book handles fame and power very well. Unbalancing if the game isn't focused on it though.
Other less-inspiring titles...
SHIP BOOKS: These are all solid. The Fighting Ships one covers vessels so large as to be unnecessary for your standard Trav adventure. A navy campaign definitely needs it though. The Traders and Gunboats is my favorite since it has some of everything.
760 PATRONS: It's solid in that it delivers on its premise. But that's fairly weak. Just listing after listing of NPC's with possible missions. It has a goal and succeeds, it's just not a very lofty goal.
SCOUT: Careers are solid. System generation is solid... the rest of the book you can take or leave so it's not a recommended purchase. Good information in there but it's only so-so in total.
SETTING BOOKS: If you've got one (3rdImp), then you're good. Just so you understand the setting. Spinward Marches is excellent as is Aslan's Trojan Reach. Unfortunately none of the MongTrav books (mentioned above that I own) hold a candle to the GURPS sector books. (Best setting book is Gateway to Destiny though...).
MERCHANT PRINCE: I was really looking forward to this and it delivers in a couple ways, fails in others. First, the careers are good, nothing to wet yourself although that's because they're... well, merchants. Second, the Corporation rules (have yet to use them) look awesome. Really awesome. Lots of potential for a fun multi-level, metaplot game but they are very specific in their focus (you won't be using them too much in standard Beowulf mode).
The trading system adds tons of detail and, unfortunately, is broken to the point of making everyone who uses it with even a single skill level capable of becoming a millionaire on pork rinds to moldy socks. The cargo size, type charts, all that stuff is neat but the mechanics are wonked. Which, unfortunately, is a primary reason to get the book.
AGENT: IMO... unfinished and criminally deficient. The careers would be solid except no rules or even discussion of the new Mustering Out benefits. I can't believe Mongoose printed this in its current state.
Odd.. I was trying to think of others since I know I own quite a few but an internet check shows a definite slowdown in titles. That's unfortunate.
EDIT: Also just realized who was posting so ignore the beginner recommendations on setting/ships etc.