I am Legend is the first zombie apocalypse novel. It is also one of the great vampire novels of our time. As such, it is staggeringly influential book in science-fiction, fantasy, and horror. This is doubly impressive because the book is really short at one hundred and sixty pages. However, it's the book which created the vast majority of tropes we've come to take for granted in our vampires, zombies, and supernatural science fiction.
The premise of the book is fairly simple (the best stories often are): Robert Neville is the sole survivor of humanity as far as he knows. After a nuclear war was lopsidedly won in the United States' favor, dust storms cover the globe and bring with it a contagion which turns a large portion of the population into vampires. These vampires drink blood, fear garlic, hate crosses (if they were Christian in life), despise mirrors, cannot transform themselves into other forms, die if staked, sleep during the day, and possess little in the way of intelligence.
The undead, further, come in two varieties with the "living" vampires who are sick and insane as well as the "dead" vampires who are actually animate corpses. Both crave blood and Robert Neville doesn't distinguish between the two as he hides out in his fortified suburban home during the evenings while staking vampires during the day. Eventually, Robert grows tired of simply trying to eradicate the undead one at a time and teaches himself bacteriology to find a cure before events change dramatically with the discovery of a fellow survivor.
I am Legend is an impressive book from a purely literary standpoint because it manages to be 90% internal monologue. As the last man on Earth, Robert Neville has no one to talk to or interact with so he just thinks about all the various things which have happened to him over the past few years. We get a few scenes set during the fall of humanity to the undead and the scenes with the fellow survivor he finds but most of the story is done purely through Robert Neville talking to himself (and the reader).
Thankfully, the events are quite interesting and Neville's interactions with the dead world around him engaging. While many of the scenes from I am Legend are familiar, like walking through an abandoned city which may or may not be full of hungry undead at any time, that's because this inspired those scenes. We also have a lot of interesting philosophical treatise from Robert as he debates the merits of survival when there's no one left to live for.
The best moments in the book are Robert struggling with his own loneliness and how isolation has affected his insanity. It's very clear, even to Robert, that he's lost a good deal of his grip on reality. He's tempted by the undead women outside, is prone to paranoid rages, and constantly contemplates suicide. The main character is an atheist and it's interesting how this impacts his worldview in surroundings which seem so obviously supernatural but aren't definitely so.
One of the most fascinating bits is when Neville tries to scientifically explain away the undead around him. He determines that it is a combination of garlic-weak bacteria and psychological factors which create the traditional weakness of vampiredom. Matheson's vampires are among the first scientific vampires and zombies ever created. This opened the way for Resident Evil and so many other disease-based zombie stories (not the least being Night of the Living Dead).
As a novel written in 1954, there are some areas the book doesn't hold up in. For example, despite living in Los Angeles, Robert Neville doesn't really acknowledge anyone non-white living in the city. Indeed, when thinking back on a dead colleague, he refers to him as "The Negro" rather than, say, Thomas, or whatever the man's name was. Neville is also something of a wunderkind soldier, thinker, and more but I can't fault him too much since these skills are necessary to explain how he's stayed alive alone in the apocalypse.
This novel has one of the most famous twists in science fiction at the end but, impressively, it's largely still secret to modern readers. I won't spoil it but the ending is one which makes perfect sense for the story and is one which leaves a powerful lingering impression. It underscores a lot of the book's themes and is perhaps one of the few endings you could come up with to improve the emotional stakes following the apocalypse.
In conclusion, I am Legend is something which should be read by every serious horror, science-fiction, vampire, or zombies. It's a seminal piece of literature in the genre and with good reason. It's an easy read and one which can be finished in a couple of hours too. It's not perfect but definitely enjoyable even with so many of its innovative ideas having now since become commonplace.