A Christmas Carol is one of the stories in the Selected Illustrated Works of Charles Dickens, which includes the Christmas stories, ghost stories and other tales. Many know the story about the miser that despises Christmas and learns an important lesson when visited by the Ghost of Christmas past, present and future. The story takes place during Christmas but it really is about other things. It's about the disadvantage of capitalism, which robs people of their human emotions, where selfishness and indifference take the place of humanity and generosity.
Another theme could be faith. In the beginning, Scrooge has no faith. He is a cynic, dismissing everything as "humbug", but after getting a new perspective of himself and the world, he turns into a believer.
Something worth mentioning about Dickens work is the occasional half-digressions that aren't necessary for the plot. The descriptions are beautiful and colored, but demands patience from the reader and, to a certain extent, actually slow down the pace.
During the Victorian era, ghosts were common in literature. If not as scary beings, they served to deliver a message, explain things, present the reader with feelings, emphasized with the supernatural element. In A Christmas Carol, the ghosts could be real or not. Ebeneezer Scrooge could be a man wakened by his conscience, hunting him in his sleep. Or he could actually be visited by those ghosts. Either way, the ghosts aren't important. The message about human greed and ignorance is the important matter, and that every man can choose to improve, it's never too late. And, that happiness doesn't come of money, but of humanity.