1951 book by Italo Calvino, Italian author. Published originally in Italian as "Il visconte dimezzato".

Like the best fantasy literature, this book (read by me as part of the "Our Ancestors" trilogy, alongside the (almost)equally entertaining "The Baron in the Trees" and "The Nonexistant Knight") works on several levels: the first, the most immediate, is that it's just a well-told story with some entertaining plot twists and memorable characters. If we dig a little deeper though, we find there is more at work here than just storytelling.

Gabriel Garcia Marquez once described the problems he faced in writing "One Hundred Years of Solitude": he had the idea, the plot, in his head, but he couldn't find a way to tell the story believably; then, it suddenly struck him to tell it as his grandmother had told him: to the point, with a straight face. Basically, by telling supernatural stories as real, your audience would suspend their disbelief and believe it themselves. Italo Calvino takes a similar approach here; viscount Medardo di Terralba (the "Medardo" suggesting "mezzo"/"medio" or "middle") is fighting alongside the Christians against the Turks. This war is just a backdrop, but it is at this war that Medardo is shot by a cannon and ripped in two parts.

Fast-forward to his return to his hometown of Terralba, where his nephew and our narrator (unnamed) lives, and is currently about 8 years old. Medardo returns, cloaked and dark. Only his right side has returned and soon we realize this is where all the evil in him is concentrated1 - he goes through the countryside, cutting everything in half, leaving a trail of destruction and depressing his father to the point of bringing on his death.

Rather than stray into plot summary for a book that is truly worth reading, I'll just mention that indeed the other side does return, and the ensuing conflict reveals the central theme of this book - that not everything "evil" is bad and not everything "pure" is good - in other words, an attack on Maniqueism, the notion of only absolutes with no gray area.

Even if you're not much given to fantasy literature, the fluid storytelling, unpredictable love story (and I used to think that was an oxymoron) and just a generally entertaining experience make this a book worth reading, and if you happen to pick it up with the other two parts of the trilogy, even better.

[1] Read into the right/left evil/pure choice as you will, keeping in mind that this was after all soon after World War II, and that Calvino demonstrated leanings towards Communism at several points in his life, in fact being part of the Italian Communist party until events in Hungary around 1956 motivated his leaving of the party.