Robert Neville may well be the last living man on Earth . . . but he is not alone.
An incurable plague has mutated every other man, woman, and child into bloodthirsty, nocturnal creatures who are determined to destroy him.
By day, he is a hunter, stalking the infected monstrosities through the abandoned ruins of civilization. By night, he barricades himself in his home and prays for dawn....
First of all, the movie with Will Smith is a cool movie. I dug it. But... it is so loosely based on Matheson's work you can barely call it an adaptation.
I Am Legend is a fantastic read. The vampire virus (to which Robert Neville is immune) is plausible, and Neville's plight is heart-wrenching. The ending of I Am Legend has to be one of my all time favorite endings.
There are a couple of movie adaptations that pre-date the Will Smith version. One of them is a great adaptation starring Vincent Price called The Last Man on Earth. This adaptation follows closely to I Am Legend. If you haven't seen The Last Man on Earth, I highly recommend you read I Am Legend first. While the movie includes Matheson's ending, it fails to capture the same depth and meaning that Matheson conveyed.
If you are a zombie fan - especially a George Romero fan - or a fan of the vampire, I Am Legend is a definite must read, but I also highly recommend it in general.