I don’t have any superior intellect or education to qualify me to comment here. This post did, however intrigue me. I think it is an interesting question, and one I have never thought to ask.
In regards to whether it is a sin to write stories of the devil, I would say no. Most of the ideas we have about satan come from extra-Biblical texts; Scripture itself holds very little detail about Lucifer. I have never heard anyone speaking on Catholic doctrine proclaim that Milton or Dante had sinned.
Now, can Satan tell the truth? This fascinates me. We know that the only thing God can not be is that which is contrary to His nature. God can not be a liar or a sinner or hateful, because it is impossible for God to lie, sin, or hate. I don’t know that the boundary is so clear cut with Satan. Satan is the great deceiver, and is certainly capable of stating fact. So if you mean “truth” to be an empirical statement of fact, I would respond “yes, absolutely”. If you mean “Truth” in the spiritual sense, I would say no, because it would be completely contrary to the essence of his very being, since Truth ultimately leads to God.
I also think that he is absolutely capable of presenting fact as a false representation of Truth if it suits his own ultimate purpose.
