It looks like there might be a typo or a mix-up with the title or author’s name—there is no widely published book titled ***Nineteen Quiet Rooms* by Ottilie Marsh**.

Could it be one of these instead?

* ***The Quiet Tenant* by Clémence Michallon** 
* ***Nineteen Minutes* by Jodi Picoult**
* A mystery by **Ngaio Marsh** or **Katie Marsh**
* Another "locked-room" or numbered mystery (e.g., *Eight Perfect Murders*, *The 7½ Deaths of Evelyn Hardcastle*, *The Guest List*)

If you can double-check the exact title, the author's spelling, or mention a character name/premise you remember, I can give you the actual ending in time for your meeting!