Tangible Ghosts is a difficult book to pigeonhole.
It's a murder mystery and an espionage thriller, set in an alternate history, with environmental overtones. The book doesn't quite succeed on all these counts, but the alternate history is deep and interesting enough to make the book worthwhile. Modesitt posits a world in which ghosts are a real electromagnetic phenomenon, created when humans die in great stress, fading over time. Conquest is a slow process because the ghosts created by war render territory uninhabitable until they fade. Hungarian empire to develop ghost-removal technology.