Okay /tg/, it is the gift giving season.

I'm planning on purchasing some traveller books for my girlfriend, as they are not only awesome books, but she wants to GM a campaign or three using those rules.

She has the Mongoose reprint of the core rules, so the question is this: where to now? Mongoose has the first two books, first two supplements, and Spinward Marches published, and I have no idea which is the best bang for my proverbial buck.