The first book in the Mechanoid Invasion series.
From the inside cover:
"The Mechanoid Invasion is the first chapter of an epic intergalactic trilogy. Each subsequent game will pick up where the other left off, but, as with the Mechanoid Invasion, will be complete in itself."
The first RPG by Kevin Siembieda, the Mechanoid Invasion took place on a colony world under siege by alien invaders. The game included rules for player classes ("Occupational Character Classes"), combat, vehicles, a native alien race (the Rovers), and several types of enemy mechanoids.