Often fiction books are criticized for diverting people away from truth/reality. Some people have gone so far as to say that fictional stories are lies. Let us think about this rationally for a minute. Fiction can be used, either for good, or for ill.
I will give an example of what I mean in just a second.
If a person reads a story about strength, (like say Superman) it may inspire them to become better people. So, while the story of Superman is not ITSELF a real story, it does reinforce a real quality that all people should aspire to, strength.
An example of where fiction can be BAD is if it is written in such a way as to mislead the reader. One example of this are various tabloids. Some, it is blatently obvious they are writing bovine fecal matter, others, it’s not always so easy to discern.
So, is fiction dangerous? Depends on whether it gets into the wrong hands.