Arguably, he was the most prominent and well known film critic in America. He was the first film critic to win the Pulitzer Prize (in 1975) while he was writing for the Chicago Sun-Times newspaper.
1975 was also the year he began filming the PBS movie review show Sneak Previews with rival Chicago film critic Gene Siskel. They continued reviewing movies together on television until Gene Siskel's death in 1999.
Roger Ebert's movie criticism is often as entertaining as the movies he's writing about. One of his most famous reviews was this, for the movie, North:
"I hated this movie. Hated hated hated hated hated this movie. Hated it. Hated every simpering stupid vacant audience-insulting moment of it. Hated the sensibility that thought anyone would like it. Hated the implied insult to the audience by its belief that anyone would be entertained by it"
Including reviews of some of the greatest movies he has ever encountered, this book not only has wonderful commentary but is a great viewing guide if you're looking for a couple of wonderful movies to check out.