Exactly how steamy is this story?
R, or even X
It's tough to give this novel a rating because it has no overt scenes of sexuality. Sexual acts are described in the vaguest of ways, often with Latin terms standing in for clear depiction. If you look up the Latin words in a dictionary and read really closely, you do get a sense of the deepness of Humbert's perversions as well as the various sex acts in which he and Lolita engage.
The novel warrants an R at the very least for its illicit subject matter (though the author of the Foreword would disagree, claiming that sexual scenes are "functional" and not "sensuous," moral and not arousing). The novel includes rape, incest, pornography, prostitution, and sex of all kinds. According to IMDb.com, the 1962 movie version did not have a rating, but it included warnings that "A girl seduces her stepfather to have sex with her." The 1997 version was rated R.