I have to be honest with you: **I don't have specific knowledge of a novel titled *Nineteen Quiet Rooms* by Ottilie Marsh.** 

Since book club is tomorrow, I don't want to make up a twist or risk giving you totally incorrect information that would ruin your discussion or confuse you. I cannot find this title or author in my training data, so it's likely a very recent release, an indie/self-published title, or a book with a title that's slightly different from the one I have access to.

**However, I can still help you save your book club tomorrow! Here are a few options:**

1. **Tell me what you've read so far.** If you give me a summary of the plot, the characters, and the last chapters you finished (the setting, any weird clues, the protagors), I can help you brainstorm likely twists based on classic thriller/horror tropes (like a house that isn't what it seems, a character who isn't real, or a time loop). 
2. **Quick spoiler search:** Since the book is likely very new, try searching the title + "ending explained" or "spoiler" on Reddit (r/books or r/horrorlit) or Goodreads. Readers usually post detailed summaries of endings there within days of release.
3. **Ask a friend in the club:** If you have a friend who finished it, ask them privately to just tell you the twist before tomorrow. 

If you want to give me a rundown of the plot you've read so far, I’d be happy to help you analyze it and predict a plausible ending so you don't go in completely empty-handed!