It is without a doubt illegal. Laws are in place to protect children and prevent the spread of any material that could potentially harm them or contribute to the normalization of inappropriate behavior involving children. Reading child porn fiction falls under this prohibited category.
I wouldn't recommend looking for such content as it's often inappropriate and may be illegal. You should focus on more positive and legal forms of entertainment.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.