The Stainless Steel Rat Wants You (Stainless Steel Rat Series #4) Book

After saving the world, diGriz is called on to save the universe. Liberating his two, now teenage, twin' sons from a military boarding school and penitentiary, diGriz sets out to free his wife, who has been arrested by the tax men. But the family is soon fighting an enemy of a different sort, when the humans-only galaxy of the League is invaded by all manner of hideous aliens.
The Rat, disguised in the most hideous combination of alien physical features, is sent into the centre of the aliens' stronghold, where he finds himself the object of desire among the aliens. His task is to stop the aliens, who plan to wipe out every human in the universe.