Lial'kovody - Robert A. Heinlein

It is the summer of 2007. The sun is shining, music is dancing on the air, and kids are playing in the streets. What they don't know, though, is that slug-like creatures have come to Earth in flying saucers, attacking the world and its people. By attaching themselves to people's backs and taking control of their nervous systems, the slug's use humans as puppets with the intention of taking over the whole world. Earth is quietly thrown into chaos-but a few special agents could save them all. Sam, Mary, and The Old Man-the head of the clandestine national security agency called the Section-travel to Iowa to investigate a report involving a flying saucer as well as the disappearance of six agents previously on the case. Learning that the city is being overrun by slugs, they are disheartened to learn that they can't convince the US President to officially declare an emergency. The three are attacked but manage to escape. However, a slug is able to escape with them. The situation becomes more dire when Sam gets taken over and the slug starts using Sam's vast skills and knowledge to facilitate the invasion. Is this the beginning of the end for the human race?