The Sanatorium by Sarah Pearse (Random House UK)

Old sanatorium in Swiss Alps modernized into a grand hotel. Elin's estranged brother invites her to celebrate his engagement with her childhood friend Laure. But there are deep resentments and suspicions between siblings, the death of their younger brother is still lingering in their thoughts. And above all, people are found dead, with strange masks on their faces... 

What a marvellous and thrilling novel! And the atmosphere. I love it!

4/5