Literature can be used to gain “reliable knowledge.” There are ways in which the knowledge can be viewed as having or lacking certainty.
In the case of a piece of literature with a first-person narrator, the situation is a little bit different. First-person narrators are not necessarily trustworthy and can, at times, manipulate the story.
Offered, the narrator of Margaret Atwood’s The Handmaid’s Tale, mentions in a number of places in the novel that she has manipulated the story.
Even with a narrator like Offered, a reader can gain reliable knowledge from the novel. The degree of certainty is debatable.