Traveller is a game set in a universe that looks somewhat Star-Warsy, if you aren't aware of or haven't read the decades of science fiction that came before 1977, on which the game is actually based. There is no one source; the game has many fathers, from Jefferson P. Swycaffer to Gordon Dickson to H. Beam Piper. Heretofore there hasn't been a compendium of sources, or discussion of influences, or reviews of how these books and authors came to be part of the Traveller background. That's where Shannon Appelcline and "The Science Fiction in Traveller" comes in.
The title refers not so much to the technologies and SFnal elements that go into Traveller than the way these technologies shaped the galactic societies in which they are placed, and how those elements appear in Traveller. For instance, Dickson's "The Genetic General" (known better today as "Dorsai!") does not much resemble Traveller in terms of tech. But it does have a Mercenary's Code, a feature that could be useful for Game Masters looking to provide more detail and substance to their own Traveller merc campaign. H. Beam Piper's "Space Viking" doesn't seem very Traveller, with its half-primitive space raiders devastating planets for the tech and raw materials to sustain their own civilization... but the home worlds of the Space Vikings have the same names as the Sword Worlds in Traveller, and the use of hyperdrive jump starships and their effect on commerce and warfare is very close to what you find in Traveller. Here too a GM can find inspiration for a campaign, raiding worlds for supplies or defending against them. In each case you find a little more of the Traveller universe and a good deal more in terms of usable background, story organization, campaign goals and structure to enhance your Traveler campaign.
My own long-time Traveller GM made much use of Keith Laumer's books on Retief, the diplomat to the stars, which greatly informed his campaigns with political and diplomatic subtlety -- even as we poor schlubs were using air/rafts to break into secret prisons for hostages in an enemy capital, hoping to get away clean. It's this sort of backgrounding that makes a campaign feel more real, and greatly enhances the player's enjoyment of the setting as well as the adventure. Bigger Things Are Moving In the Shadows, and if you are lucky, you might get to find out how to profit for them.
If that's the sort of thing you're looking to do for your players, I heartily recommend "The Science Fiction In Traveller" as a way to discover the roots of the game and the stories that can inspire world-spanning adventures for your players. Do look into it; you'll be very glad you did.