I consider the classic Meltzoff cover - on American paperbacks of this for the first 20 years - to be perfect for the book. When I started reading and collecting Heinlein, this is what they looked like, and even after I got the titles in hardcover - the early Signet paperbacks remain the standard of how I think a Heinlein book should look.

The Puppet Masters (1951) - serialized in Galaxy September to December 1951. "Lock your doors! Never enter a dark place! A man wearing a coat is an enemy- shoot!" The compelling and paranoiac tale of parasites who invade Earth - able to take over a man and have access to all knowledge and abilities of the host, and intent on spreading out and taking over the planet. Our hero is a tough agent of a super-secret intelligence agency, called in to check out an alleged "flying saucer" landing in Iowa - knowing that the first 4 agents sent to check didn't come back. With his boss and another agent, they uncover one of the slugs - but who will believe such a fantastic tale? The slugs move fast, and soon it’s a fight to the finish, still hampered by the unwillingness of people to believe - for when the slugs take over a city things seem to run just the same. Heinlein gives us competent, fast-thinking heroes, who have to scramble even if they make all the right moves - just to have a chance for survival.

This one grabs you and takes you on a fast and exciting ride. The Puppet Masters is the book I credit for turning me from a casual reader of SF (among other things) to a fanatical SF fan and collector - when I first encountered it at the "golden age" of 13. Highly recommended!