He’s also a big name in science fiction. Though he’s been publishing short fiction since the 90s, it was the release of Old Man’s War in 2005 that really put him on the map. Old Man’s War is pretty solidly military space opera, and it’s often compared and contrasted with the seminal novel Starship Troopers by Heinlein. The basic premise is that mankind has entered the stars, but the galactic neighborhood is both crowded with many other sentient species and violent with conflict. Lacking most of the egalitarian and positivist attitude of Star Trek, instead the Universe is harsh and deadly and mankind must do whatever it can to survive. Even with this not particularly unique setting (not too dissimilar from the Uplift universe), the way that Scalzi handles it is interesting. All humanity outside of Earth is controlled by a military hegemony, where all resources are dedicated to finding and eliminating threats from antagonistic alien races, and expanding human colonies to new systems and planets so that the loss of any one system doesn’t threaten the entire species. However most of Earth lives their lives not too differently than people do today, people’s eyes aren’t really opened to the rest of the universe until they choose to enlist and join the military, with some very interesting and unique terms of enlistment that also tie into the novels title. Following the protagonist we find out that humanity is sitting on extremely advanced medical and bio-technology used to make super-soldiers that can fight the hostile alien menace. And our protagonist must come to terms with the vastly different society created by a space-bound and technologically advanced military and society that has eliminated things like disease and old age, but has definitely *not* eliminated death by violence.
However the first book of his I would recommend is Lock In. It’s a more down-to-earth near-future science fiction story where a decade before an incurable disease left millions of its victims completely paralyzed but otherwise alive, also known as lock-in syndrome. A Manhattan Project-style huge research initiative wasn’t able to find a cure, but was able to create direct human brain to computer interface that allowed victims of lock-in syndrome to communicate with the outside world, and later control prosthetic artificial proxy bodies known as ‘threeps’. As the technology has been refined lock-in patients now live out their whole lives through their threeps: going to school, holding jobs, falling in love, etc. The story is about a lock-in patient who becomes the first locked-in police detective, and gets involved in a high-profile murder case involving lock-in patients and threeps. In addition to being a good whodunit, it raises interesting questions about the nature of self, handicap, and how we treat those different than ourselves.
And if you’re into humor/parody in your science fiction, Scalzi also has a great standalone novel called Redshirts. In an extremely thinly-veiled pastiche of the Star Trek: The Original Series, we follow the life of some of the crew members with short life expectancy: the red-shirted Ensigns that always invariably die on away missions. In much the same way that Galaxy Quest is the best Star Trek movie even though it’s not actually a Star Trek movie, Redshirts is by far the best Star Trek novel that’s ever been published.