I don't think this was the first 'mind controlling alien invasion' story, but it's the best. Earth is attacked by slug-like creatures, who infiltrate important sectors of society, take over communications, and then expand at an exponential rate until the entire population has been parasitized. The midwest falls in weeks, Russia is taken over almost immediately, and it's up to super-spy 'Sam Cavanaugh' to foil the invasion.

There's a lot of action and excitement, as well those Heinleinian world-building moments, like the dense, post-nuclear war cities, or the 'not a sparrow-fall' radar network. The puppet masters are an appropriately horrifying enemy, and one as scary as anything short of a Berserker/Reaper-esque extinction machine. Sure, the gender politics are pretty retrograde, but that doesn't detract from a tight, terrifying, and damn near perfect scifi adventure story.