I would like to take the opportunity to nominate "The Invaders Plan" by L. Ron Hubbard
The Voltar Confederacy has a long-range strategy to invade Earth and use it in their conquest of the Galaxy. However, with the discovery that Earth is being destroyed by incessant pollution, Royal Officer Jettero Heller is sent on a top-secret mission to handle this threat to the planet's life. But why is Lombar Hisst, head of the Apparatus, Voltar's deadly intelligence agency, determined to sabotage the mission and see it fail? Can Heller possibly succeed or will he fall into the web of intergalactic intrigue spun by Hisst and his devious henchman, Soltan Gris? Find out as you embark on this mission full of dynamic characters and packed with plenty of twists, action and emotion.