Advice on next book purchase(s)

[Michele]

Originally Posted by Hohenstadt

What would you all recommend, after those, to round out my collection as to be able to do any RPG setting?

Any setting is one thing. But the question would be, what genre?

I mean, GURPS Mysteries would be my suggestion, and it can work even in fantasy settings, not to mention futuristic, modern-day, horror etc. But if you and your players don't like investigations, puzzles, and, well, mysteries... there's little point.
If your gaming group likes non-combat interaction with a small number of fully fleshed out NPCs, then GURPS Mass Combat is of limited usefulness. And so on.