I wish I had been able to read this in the original French, but alas! I had to settle* for the English translation available at my library. I felt it was quite well done, but I could only think of how lovely it probably was in French.
Really, nothing prompted me to pick this up other than it being on the list of 1001 books you must read before you die (which I take with a grain of salt, mind, but it is helpful for picking out things I normally wouldn't), so I began with no presuppositions, except that it is about ... an immoral person, I suppose. The reviews here that state that the narrator, Michel, is not immoral strike me as totally bizarre. "Well, it's not like he killed anyone!" they complain. "More sex! Where are all the homosexual sexytimes I expected? I want my money back!" they yell. So ... immorality is only murder, or only the explicit description of homosexuality or pederasty?
The Immoralist really isn't about immoral acts. It's about an immoral person. Immorality, here, is not just the usual suspects of murder, rape, adultery, et cetera. It has to do with how you interact with others. Michel, the narrator and protagonist, marries Marceline in order to please his father and basically because he can't think of anything else to do. He's driftless, spineless, colorless, and completely out of touch. After a particularly nasty attack of tuberculosis on his honeymoon in North Africa, his wife (the saintly, wronged woman archetype) nurses him back to health and tries to amuse him by introducing him to the local children. Michel takes an interest (ahem) in one of the boys, whom he finds youthful, beautiful, full of life, and intoxicating. He resolves to really "live" life. He embraces sensuality, but not in the way we'd normally think of it. Once he gets tired of looking at a landscape, he moves on, heedless of what it's doing to his wife. Once a boy becomes too like a man, he is no longer attractive to Michel and becomes repulsive.
We see how Michel treats Marceline--it's immoral. We see how he casts people aside because they no longer amuse him--that's immoral. He's totally self-centered, which only becomes more clear as he desperately attempts to convince his audience that he truly loved his wife, only lived for her, cared for her ... sure you did, Michel. So you say.
Other commenters note that Gide is influenced by Nietzsche and other philosophers of that ilk--I've not read any of that nor do I have the desire to, so I may be missing that aspect of this little story. I can't say I loved it or that I'd read it over and over again, but some of the prose is downright brilliant--the kind you want to copy out and read when you feel particularly meditative. Recommended.
Afternote: *settle being that at the time I read this, copies on Amazon were like wayyyyy more than I could afford to pay to get a French copy, because evidently the French publishers are Raking It In on book prices. Actually, this was true even in France, where books were crazy expensive and I bought secondhand as much as possible, being a poor teacher/student/temp worker. Someone asked me (snarkily, I might add) on Goodreads why I just didn't read it in French. J'étais sans un sous, madame.