The Independent Command (Star Trek: Flight Engineer, Book 3)

When the spiderlike Fibians launch an attack against Earth and its colonies, flight engineer Peter Raeder takes over command of the starship Invincible from the ship's injured captain and journeys behind enemy lines to stop the raiders, but he soon finds himself lost in alien space, with a damaged ship, and discovers a strange secret about the monstrous Fibians. Reprint.