Throughout the first and second seasons of Game of Thrones, the White Walkers are considered to be dead by everyone but the wildlings (and the "deserter" Ranger in the first episode). For example, there is a scene where Maester Lewin dismisses Osha's words about the White Walkers "only sleeping".
Now, we know that Commander Mormont knew that the White Walkers were alive and kicking, since he knew what Craster did to his sons (he confirms this in What is Dead May Never Die). I can't think of any scene where he dismisses the idea of the White Walkers existing, but he certainly never mentions anything about the existence of the White Walkers. None of the men of the Night's Watch seem to believe in the White Walkers (when they let the wights in, and the numerous times when Sam talks about them).
So, my question is, if Mormont knew about the White Walkers, why didn't he tell anyone?
(I don't mind answers based on the books instead of the TV show)