I think the story book might be located in a special vendor's shop. Sometimes, these unique items are sold by specific NPCs. Or, it could be a reward for completing a challenging side quest.
One idea could be a rengoku fanfic where he trains a new group of young slayers. It could focus on his unique teaching methods and how he instills his values in them.
Tanjiro would be a great character to pair with rengoku. Their different personalities could create an interesting interaction. Tanjiro's kind - hearted nature and rengoku's fiery determination could lead to some great storylines.