Alright, so this is odd - Goodreads and Booklikes and a few others list this book as "The Secret Rooms: A True Gothic Mystery" while Amazon and the library and other places list it as "The Secret Rooms: A True Story of a Haunted Castle, a Plotting Duchess, and a Family Secret."
The story flips between, firstly, the author's experience navigating the Duke of Rutland's family archive of letters for a book about the effect of the Great War on the local population which quickly changes to uncovering why there are missing sections of documents in this reportedly perfect record?! and, secondly, the events that occurred within the Manners family during and around the missing periods of time. The main character of focus is the 9th Duke of Rutland, John Manners whom was responsible for creating the archive originally. The mystery is narrowed down by the author to three missing sections of time in the records and she eventually uncovers enough evidence to recreate what likely happened.
An interesting story, but the reprinting of letters in their entirety multiple times towards the end of the book to show how all the pieces come together became very tiresome, very quickly. Otherwise I quite liked it.