The Distant Hours by Kate Morton
However, it is a very good book. A beautifully written book. Just not one that you should expect a lot of quick answers and movement. If you go into it with that in mind - I think the time put into it pays off greatly at the end with a very satisfying twist ending and a lot of mysteries solved.
From Publishers Weekly:
A letter posted in 1941 finally reaches its destination in 1992 with powerful repercussions for Edie Burchill, a London book editor, in this enthralling romantic thriller from Australian author Morton (The Forgotten Garden). At crumbling Milderhurst Castle live elderly twins Persephone and Seraphina and their younger half-sister, Juniper, the three eccentric spinster daughters of the late Raymond Blythe, author of The True History of the Mud Man, a children's classic Edie adores. Juniper addressed the letter to Meredith, Edie's mother, then a young teen evacuated to Milderhurst during the Blitz. Edie, who's later invited to write an introduction to a reprint of Raymond's masterpiece, visits the seedily alluring castle in search of answers. Why was her mother so shattered by the contents of a letter sent 51 years earlier? And what happened to soldier Thomas Cavill, Juniper's long-missing fiancé and Meredith's former teacher? Despite the many competing narratives, the answers will stun readers.