Where can I read up on the Traveller fluff, is it in the rules, various supplements, stories, blogs?
The material is spread through nearly 40 years’ worth of supplements, rules and other stuff. You could start with the Traveller wiki which has a useful article on the different versions of Traveller linked from the front page as well as loads of detail that you could probably waste hours surfing through. To be honest, it’s a rabbit hole and you could get lost down there!
After that, it is a case of deciding what area of Traveller might interest you most. Classic Traveller is available for download from RPGNow. That’ll give you the background to understand the technical details of the source material, as well as an understanding of the premises behind the original Traveller game. After that there is no one specific way to pick up the background. The Journal of the Travellers Aid Society has lots of material and may be more readable than the individual adventures. The Spinward Marches supplement is the classic Third Imperium background and home to the Fifth Frontier War that formed the background to much of our early Traveller gaming way back when I was still young and enthusiastic. There’s also a board game of the Fifth Frontier War. Various of the websites linked from the Traveller wiki will fill you in with more detail too. There’s probably more free material out there too, but I can’t think of it immediately right now.
Once you step away from Classic Traveller, there’s a whole different bunch of source material for the other versions of the game. Like I wrote, it’s a rabbit hole. 🙂