I haven't come across a good 3I book, yet. GURPS Traveller (the core book for GURPS 3e) provided a pretty solid set of cleaned up library data entries with several explanatory side bars explaining the hows and whys.
I've said elsewhere that 1248 is the only set of products that clearly lay out all of the Traveller meta-plots and does something with them. Everything else deals with the 3I sort of as a not quite articulated default or else as something you interpret from a mass of contradicting data points. Or even... you learn about the 3I by learning how it contrasts with the Zhodani, Hivers, etc.
Historically, traveller referees have their own interpretations of various bits and pieces from the Classic Traveller rules and adventures. They weave them together into their own vision of the 3I... and none of them are in sync with where GDW actually went with the universe.
That said... for referees, I suggest the CT "books", the CT adventures, the CT alien modules, the various references to the Spinward Marches in the early GT books and the early SJG-JTAS articles... and Rim of Fire and GT:IW for more background details.
For players, I would avoid having them drink from the fire hose. They need to have a feel for the broad strokes of what's going on in their subsector. From there... feed 'em library data and TAS entries as needed to flesh things out. Let 'em dig into the GT core rule book if they want a *really* high level picture of the setting.