The Puppet Masters by Robert A. Heinlein

I think this book was later made into a movie, which I never got around to seeing. It will probably turn up on a discount rack someday and I'll grab it. The story borrows liberally from H.G. Wells tradition of alien invasion. A flying saucer lands in a small town in Iowa, and three agents of a super secret government agency - even the CIA doesn't know about them - arrive to investigate. When they get there, the media is saying that it was all a hoax perpetrated by some local boys, and those self-same boys are giving "tours" of the obviously cobbled together saucer.

Something doesn't quite smell right about the situation, though, and our protagonist, Sam, his fellow agent Mary, and the head of the agency, known as The Old Man, are determined to get to the bottom of things. They discover that slug-like aliens from Titan have arrived in large numbers, and are able, by attaching themselves to the spinal cords of humans, to direct their actions like puppets. The Old Man manages to eventually convince the President that something is amiss, but Congress proves to be a bit tougher, and by the time the danger is recognized, vast swaths of the country are under control of the masters.

Interesting side note, Heinlein at one point describes a map in the Pentagon showing the areas of alien control, and it sounds eerily similar to the blue state / red state map from our 2008 elections. Hmmm.

Sam is sent into occupied territory to try to obtain a specimen of the slugs for the scientists to study, and is himself taken over for several weeks, participating in recruiting new puppets. He is rescued, eventually, and the master is removed. Sam is debriefed under hypnosis, which reveals a great deal about the aliens' habits and plans, and the scientists are trying to find something that they can use to kill the slugs without killing their hosts.

Predictably, and with the usual Heinlein attitudes about women, Sam and Mary fall in love and get married. Their honeymoon is cut short when the aliens have staked out Sam's mountain cabin retreat, and Mary is briefly possessed by one of the slugs. Rather fortuitously, it turns out that Mary has her own earlier history with the Titans, as her family was part of a colony on Venus that disappeared, and her childhood memories contain the necessary clues to get rid of the aliens once and for all.

Nothing surprising, and nothing really new in this story. It's a pretty quick read, though.