Hermione, Ron and Harry do find a reference to it in Magick Moste Evile, which was in the Restricted Section - but it merely says, "Of the Horcrux we will not speak." As Hermione (I think) says, why even mention it? But it sounded like Tom Riddle at least had a vague idea as to what they were when he asked Slughorn about them... he obviously knew that they were something that could enhance immortality, which was what he wanted.