Welcome to Battlefleet Gothic, the game of spaceship combat set during the Gothic War in the 41st millennium: a grim time when the Inperium of Mankind battles for survival in a hostile galaxy. Battlefleet Gothic allows you to command fleets of warships in deadly conflict among the stars, though whether as Mankind's saviour or its destroyer remains to be seen.
The first section of Battlefleet Gothic details the core rules of the game - the nuts and bolts of how different ships move and fight in space. Later sections cover the history of the Gothic War and how to fight a series of battles as part of an ongoing campaign.
This game would have been great with more support and a time-limit on games. The system with plastic ships was inspired and led to a very wide variety of classes.
BFG is currently my favorite rulebook of all time. It has two things in its favor.
One, it's a particularly nicely executed Warhammer 40k setting, and being a middle-class British male I have a childhood softspot for 40k. The black-and-white artwork remains stellar, and the flavor boxes scattered throughout are some of GW's best.
Two, the rules are beautiful. The way blastmarkers handle so much book-keeping without needing so much as a pencil is beautiful. The way ships are balanced so each ship will only knock down the shields of another ship of the same class, so focus-firing is an emergent necessity without being specified in the rules is beautiful. The fact that ships have a maximum turn rate and a minimum speed is sublime (and why has nobody copied this yet?).
I will happily admit to being a little blind to its faults - the turret system is unintuitive, the Ork ships are somewhat uninspired, and nova cannons suffer from all the usual problems of guess-range weapons in tabletop situations - but that's because so much of it is so good. More importantly, the rules know exactly what they want to achieve, what they want to simulate and what they want to abstract away, and they deliver on this with real character: capital ships handle like the giant barges that they are, bombers can tear even the biggest ships apart if they survive their attack run, and massed weapons batteries roll satisfyingly large handfuls of dice as they blast their targets to pieces.