I've just had a look through my old sci-fi books collection and ofund a couple of series that would adapt very nicely into Traveller with very little in the way of modification.
First off, there is the Family D'Alembert series by EE "Doc" Smith. The universe as depicted in the books bears some resemblance to Traveller, in that there is an empire and there are feudal titles that are responsible for different territories. The technology is somewhat on the level of the default Traveller setting, and there would only need to be minor tweaks (possibly allowing small craft to be starships - EE Smith is not too clear on this point). Allowing the players to run agents for Imperial Security gives a clear "in" to the game.
Secondly, there are Alex Raymond's Flash Gordon novels. They are very different to the classic Flash Gordon comic, and take the story beyond Earth and Mongo. The emphasis is not on the technology, more on the action, so they could probably be done with any set of rules.
Golden Age sci-fi would give a few more options, but that wasn't the original question.
