I can't say enough about this, possibly the best GURPS book I have read, if not one of the best RPG supplements I own.
This book contains everything you could possibly want to know about creating a space-based campaign in GURPS. Everything is in here, from low-tech Saturn V rockets to living metal spaceships - everything a GM could possibly want from space opera to hard science. Even if you don't play GURPS, this is still a good purchase. Much of the information is non-specific meaning that it will fit into any campaign in any game system with a little pruning.
In short, buying this is a decision you will not regret. Not only is this a must for GURPS GMs, it's also a great read. All you need is an imagination.