The novel may be set in the late 1970's, but Neville is very much a 1950's Everyman figure. He dwells in a suburb of Los Angeles with a wife and a daughter. He carpools to work with a neighbour. He drinks a lot of cocktails. But a vaguely defined war in which Neville himself served overseas may have unleashed the disease that caused all those stories of vampirism in Eastern Europe during the 18th century. In the now of the novel, which we join in media res, Neville is alone but under siege by multitudes of vampires every night. He's turned his house into a fortress. And every day, he drives around pulling vampires out of their hiding places and staking them to death.
For a short novel (maybe 70,000 words soaking wet), I Am Legend is packed with heady goodness. There's the characterization of Neville, who reveals hidden depths as we spend more time with him. There's Neville's scientific approach to understanding just how these vampires work, and what works against them, and why. There's the back-story of the fall of society, with mass graves and an incompetent government and growing paranoia.
And there are the vampires themselves, split into two groups: living vampires who've been infected and changed, and dead vampires who continue to be animated by the contagion. Both die when you stake them, though the second group occasionally disintegrates. Neville's quest to understand what's going on in a scientific sense helps him to hold off the encroaching loneliness. He's Robinson Crusoe with a microscope and no Man Friday. He doesn't even have a parrot to talk to. But he does have his books and his classical music.
The list of later works with I Am Legend's DNA in them could probably fill a book. From Matheson's succinct glimpses of plague-fueled societal breakdown come World War Z and The Stand and so many others; from the monsters whose origins seem to be scientifically explicable come legions of infected zombies and vampires whose blood teems with bacteria or viral loads instead of magic. It's the loneliness of Neville that hasn't been replicated that often in subsequent works of horror.
Thankfully, there's also grim humour throughout the novel, much of it supplied by Neville's dead but lively vampire neighbour Cortman (!), who yells endlessly at Neville by night but whose diurnal hiding place Neville searches fruitlessly for by day. Good old Cortman. He never shuts up. Highly recommended.