Before it was a movie, it was a novel by Patricia Highsmith. It's the first of a series of novels featuring the Tom Ripley character. It's a wonderful book, full of beautiful and romantic descriptions of Italy. The character study of Tom Ripley is incredible. Not just in the exploration of his character, but in his adaptaion of someone else's personality. You get to see his need to become someone else, his inablity to function as himself before and after becoming Dickie Greenleaf. And it asks a question that has intrigued writers and movie makers for years: Is it possible to commit the perfect murder?
The movie differs from the book in many of the important details, but the ending is just as surprising and exciting. The movie also explore Tom's sexuality, and his sexual feelings for Dickie Greenleaf. In the book, the homosexual aspect of Tom's character was only briefly touched upon. All in all, both the movie and the book are outstanding and I highly recommend them both. But my rule is: read the book before you see the movie.