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.
Giyu survives in the manga. The plot doesn't lead to his death. His character has many adventures and developments that keep him alive and active in the story.
Their relationship can be developed through shared experiences. For example, if they both get injured in a battle and have to rely on each other for survival while waiting for help. This would force them to open up and communicate more, breaking down Giyu's usual stoic exterior and allowing Shinobu to see a more vulnerable side of him.
A plot could revolve around a mystery illness that is affecting the Demon Slayer Corps. Shinobu, being the expert on poisons and medicine, tries to find a cure. Giyu gets involved when he discovers that some of his comrades are showing symptoms. Together, they suspect that a new type of demon or a hidden enemy within the corps is causing this. They have to investigate, and during this process, they face many challenges and also grow closer as they share their concerns and ideas.
The Eight Fairy Slayers was a modern novel written by Nine Yin Fighting for the Stalk. The storyline and character introductions of Celestial Bane were not clear. It might be a novel with the theme of Xianxia, which included eight female characters. However, the specific storyline, background, and other details could not be learned from the search results provided. Therefore, I can't provide any more information about the Eight Celestial Slaying Ladies.