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.
Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.
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.