It depends. If the graphic violence or sexual content is an occasion of sin for you, then you should avoid such novels. But if it doesn’t tempt you to sin and is not otherwise morally-problematic (e.g., pornographic), then it is okay in principle to read such novels.
Most works of art from the various artistic disciplines contain some problematic material, and sometimes they do so for legitimate reasons (e.g., realism, character development, social commentary). The concern for Catholics is to discern between that which is necessary to the creation of art and that which is included solely for the sake of the glorification of evil or for public titillation.
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