Praise be to Allah.
Reading erotic stories, even if that is only with one's spouse, involves many evils including the following:
1 – These stories are acquired either by purchasing them or borrowing them. That is not permitted because it encourages those who print them, publish them and distribute them. Allaah says (interpretation of the meaning): “but do not help one another in sin and transgression” [al-Maa'idah 5:2].
2 – These stories are only written by immoral and promiscuous people. Most of those who write them are kuffaar who pay no attention to religion, morals or manners in what they write. Reading them is a means to propagate their deviant ways and bad customs among the people, whether they realize it or not. The Prophet (peace and blessings of Allaah be upon him) said: “The likeness of a good companion and a bad one is that of the one who carries musk and the one who works the bellows. As for the one who carries musk, either he will give you some or you will buy some from him, or you will notice a nice smell from him. And as for the one who works the bellows, either he will burn your clothes or you will notice an unpleasant smell from him.” Narrated by al-Bukhaari, 5534; Muslim, 2628.
Al-Haafiz said: This hadeeth indicates that it is forbidden to sit with those whose company will harm your religious or worldly interests. Fath al-Baari, 4/410.
3 – These stories contain lies and exaggerations, and describe imaginary events which affect the reader in negative ways and make each partner less content with the other.
4 – There is no guarantee that these stories will not fall into the hands of children whose morals will be corrupted and they will be led astray or they will think badly of their parents. The parents may not be aware of that, but they will bear the burden of their children’s sin when regret will be of no avail.
For these reasons and others it is not permissible to read these stories at all. There is sufficient in what is permissible so that we have no need of that which is haraam; what Allaah has permitted gives sufficient pleasure which Allaah is pleased with and by means of which the individual and the society are protected from the spread of evil and immorality. And Allaah knows best.