If you're looking for something else besides BookAIWriter, Sudowrite is a powerful AI writing assistant that can help fiction authors at all stages of the writing process. It has tools for idea generation, outlining, writing, editing and refinement, so it can be used in many different ways. With different modes you can customize, project management tools and a range of credits, Sudowrite is designed to be a more comprehensive tool for creative people.
Another tool worth considering is Subtxt, which is geared specifically to helping writers develop their narrative structure and consistency. Subtxt includes tools like Narrative Agents, Subtxt Muse and Premise Builder that can quickly scaffold important story beats and create rich narratives. It's geared for screenwriters, novelists and playwrights, with more advanced tools to help you refine and understand storytelling theories.
If you want a more integrated writing environment, check out Novelcrafter. It's got a personal Wiki for tracking story details, several writing modes, and AI tools to help you write from start to finish. The Codex lets you track characters, locations and lore in detail, so it's good for both planners and pantsers. Novelcrafter also lets you use your own custom AI models and services, so you can use whatever tools you need for your writing.
Last, NovelAI is a subscription-based service that uses AI for authorship and storytelling. It's got tools like character visualization, interactive fiction and customizable editing tools. NovelAI's AI modules are trained to keep your style and voice, so it's good for people who want to write their own stories without being held back.