Traveller is in my opinion the best developed sci-fi setting ever. The Spinward Marches in the Reign of Emperor Strephon are the default time and place, the setting where everyone goes back to sooner or later. They are the heart of traveller.
Traveller is an RPG in a heroic setting like a traditional space opera. It is medium level sci-fi, not "hard"(as realistic as can be) or "soft"(mostly romantic) . There is plenty of room derring do and so on, but the derring do is made believable by the incredible work gone into drawing up the setting. Though little known, I believe Traveller is a work of genius and can be compared with Dune in complexity and surpasses almost any other work ever written.
The Spinward Marches are a vast hostile border between the Third Imperium and the Zhodani Consulate, a semi-civilized area of wars large and small. Here you can find worlds that are the pinnacle of wealth and technology, and backwater worlds where nature still holds sway. This volume gives an encyclopedic description. From the political structure to the numerous business interests, to the various races and ethnic groups that inhabit the region, as well as history both recent and ancient. Added to this are short descriptions of every planet in the region. For any Gamemaster planning a campaign here there is more then enough. And for those like me that don't play but simply devour information for it's own sake there is more then enough. The information here can suit for an epic campaign, or a low key adventure or anywhere in between. For those starting into Traveller Behind the Claw is one of the best volumes they can get.