A really outstanding historical mystery that combines procedural and paranormal elements. I was blown away by the mastery of two time periods, neither the author's own, of 1862 and 1957. Both were fascinating and deeply researched. Horwitz moves effortlessly from Civil War military history to civilian history of the same period, with enormous detail in geographical locations and the characters' backstories. In the 1950s, descriptions of characters doing even ordinary things, like taking a photo with a camera, were captured perfectly.
My very, very favorite part is a tiny bit toward the end, in one of the 1860s segments, in which the young boy is scared by an apparition of Ben, the 1950s cop to whom these ghosts have been appearing - so that there was kind of a weird time-periods bleeding through in both directions, past simultaneously present. Absolutely amazing.
Regrettably, the book was published posthumously and appears to be the only published novel by Horwitz, which is a great loss.