One of science fiction legend Robert A. Heinlein's earliest novels, 1951's The Puppet Masters is a great, goofy "alien invasion" tale that keeps you hooked from beginning to end. Set in the early 21st century (when space travel is commonplace by the year 2007!), the book is a first-person account by "Sam" (not his real name), an intelligence agent for a CIA-like organization, who, with his boss "The Old Man", and fellow operative Mary, discovers that parasitic "slugs" from another world have taken over the bodies of various political, business and military leaders to mount a complete takeover of Earth!
The slugs are disgusting creatures from outer space who absorb their human hosts' bodies and minds whose removal can result in death for the aformentioned hosts.
Sam and company manage to convince the President, the Senate and Congress (in some over-the-top sequences) that the threat the slugs pose is real, but by then most of the country (as well as other countries and goverments, including the still existant Soviet Union - remember, this was written in the 50s) have already been infested with the slugs. Our heroes must find a way to defeat these creatures or the planet will be lost!
Heinlein's trademark writing tropes pop up here. There's the useless politicans, the no-nonsense military leaders and the intelligent government operatives who just know what to do in a time of crisis. Plus, there's those goofy touches of Heinleinian humor (citizens must strip naked to prove they're not infected, which, in the Senate and Congress scenes, is played slightly for laughs as various politicans have to disrobe in front of their colleagues at gunpoint) and satire (US government propaganda warn people with gems like "a man wearing a coat is an enemy-shoot!"). The protagonists and the slugs are basically stick figures with no gray areas, though Sam nevertheless gains the reader's sympathy through his unflagging loyality to the Old Man and Mary (and vice versa), as well as the horrible and painful experiences he undergoes when the slugs take control of him. (The interrogation scene, with the slug reattached to Sam, is chilling.)
An influential and still exciting novel whose basic plot has been "borrowed" by various authors and filmmakers over the last five decades (think Invasion of The Body Snatchers, The X-Files and this classic 60s TV series episode, among other examples), as well as a so-so official 1994 movie adaptation (which lost the book's humor), The Puppet Masters is available online from us here as part of the Three By Heinlein collection.
(Note: Two years after Heinlein's death in 1988, the author's original, much longer manuscript of The Puppet Masters was finally published. No promises but I'll look into getting that version, which is more fleshed out in terms of characterzation and political allegory -apparently, Heinlein's original editor didn't care for this version- for the library.)