Dangerous Liaisons contains no symbol more important than the letter. However, the novel cannot really be said to contain letters, since it is entirely composed of them. Instead, it makes more sense to say that the symbolism of the letter is vital to the way in which the plot works. The action in the novel depends on two characteristics of letters: that they say something, and that they can be read.
We should be forewarned that this is a little bit obvious: the key to the lock on Cécile's door symbolizes access to her person. By convincing her to give up the key to her room, Valmont convinces Cécile to surrender herself to him.