The birdcage library / Freya Berry.

"The answers to a puzzle lie hidden within an old book. Open The Birdcage Library and let the treasure hunt begin... It's 1932 and adventuress and plant-hunter Emily Blackwood accepts a commission from Heinrich Vogel, a former dealer of exotic animals in Manhattan, living now with his macabre collection in a remote Scottish castle. Emily is tasked to find a long-lost treasure which Heinrich believes has been hidden within the castle walls. But instead she discovers the pages of a diary, written by Hester Vogel, who died after falling from the Brooklyn Bridge. Hester's diary leads Emily to an old book The Birdcage Library and into a treasure hunt of another kind, one that will take her down a dangerous path for clues, and force her to confront her own darkest secret..." - goodreads.com