We might argue that the best known works of each of these three writers employ a primary or central symbol which anchors both the plot and the meaning of the work. 

"The Raven", The Scarlet Letter and Moby Dick each use a significant and central symbol and the meaning of this symbol (the raven, the letter and the whale) is discussed directly in the text.

We might say then that the importance of symbols is nearly equal to the importance of the story/narrative itself, if not more important.