A House of Ghosts by W.C. Ryan

 This is a perfect read for the long, cold, winter nights. Kate and Donovan are two government agents in 1917 sent to a lord's manor on a remote island in the hopes of discovering who's been selling secrets to the German army; complicating matters are the actual, literal ghosts Kate sees all around her, some of whom are dead soldiers angry at the lord's arms-dealing. 

In other words, it's an Agatha Christie-type story but with a supernatural bent and a willingness to address the legacies of a pointless war.  It's spooky and serious without being grim or depressing, and does a good job of illustrating the messy complexities of everything from relationships to the military-industrial complex to grief. Add a romantic subplot, and you have a thoroughly satisfying read. I'd love to see more of Kate and Donovan's adventures.