I have tested several LLMs and platforms to (or try to) write books using AI. I must say that this is one of the most complete I have ever tried. I prepared the draft (with chapter titles and notes), imported it and voilà! Each chapter is written in sequence and each time you have full control to check/re-write it or modify parts of it.
I only used the platform for two very different projects. The first one was a technical book (about molecular simulations) and the second about a fantasy story. In the first case, I gave the general idea and all the chapter titles/notes were generated very well. Then I created the first chapter only using GEMINI, to check the writing, use of mathematical formulas (coded in LaTeX), etc. I got very impressed with the general quality! The second project wasn't a test. I created the preface, prologue, chapter notes, etc. using Claude Opus 5, and uploaded it as a DOCX document to AIWriterBook. The result was fantastic! The amount of options you can control about the style, genre, etc. is amazing, helping to focus the writing on the book idea you have.
The only two features I haven't liked so far were the image cover production (the three variants were too similar in between) and the AudioBook system (was updated today), which has only voices in English. But I'm pretty sure that other languages are in the roadmap.