Omnibus of The Puppet Masters (1951) + Double Star (1956) + The Door Into Summer (1957.) If you had to limit your library to a single volume - I would nominate this book. Heinlein may have a couple of better books, but I rank all 3 of these as in my "top 25." Each of these books gets a strong recommendation from me if offered separately - the combination is almost irresistable. Each book is worth re-reading several times, so a hardcover makes sense. My only quibble is I'm not fond of the Viskupic cover art.

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!

Double Star: serialized in Astounding February to April 1956, and Heinlein's first Hugo-winner. Out of luck actor Lorenzo Smythe is shanghaied into playing his greatest role: doubling for the controversial politician John Joseph Bonforte - who has been kidnapped on the eve of a major breakthrough in relations with the Martians. Lorenzo is a prima donna with the capability - and eventually the will - to pull it off. From the first sentence, Heinlein has you hooked. (One of my favorites.)

The Door Into Summer: serialized in The Magazine of F&SF October to December 1956, and the first book appearance was 1957. Compared to the urgent paranoia of The Puppet Masters, this book is almost elegiac or genial in tone - but just as compulsively readable. Alexei Panshin described this as "thoroughly melodramatic but very good fun." (You guessed it - another of my favorites.)

Daniel Boone Davis is an inventor/engineer, who has designed some automatic household appliances - Hired Girl and Window Willie - which were very successful. He has a partner (Miles) and a secretary-fiancee (Belle) in his company. When Dan wants to go slow on his newest idea - general purpose robot Flexible Frank - Miles and Belle conspire to take over the company and ease him out of control. Dan signs up for "cold sleep" in a drunken binge after learning of his betrayal, thinks better of it when sober, but Belle and Miles get the drop on him and make sure he takes his 30 year nap to tomorrow. Dan adjusts to the year 2000, and finds that he can still be effective as an inventor - but he discovers evidence that he had done more things in 1970 than he remembered. He runs into a fellow with a time machine and starts to believe that he will go back because he has gone back in time. He does so, with the intent of straightening out the mess he left behind. Its complicated and tightly plotted, and all comes out well.