Of Tangible Ghosts (Ghost Trilogy‪)‬

Bestselling author L. E. Modesitt, Jr. begins his Ghost Books series with Of Tangible Ghosts in a strange alternate reality where ghosts are no longer mere superstition, but physical realities.

When two real ghosts and the records of a murdered researcher enter the life of Johan Eschbach, the former spy and present professor finds himself caught with the secret of high-technology de-ghosting, and the whole world, including his former spymasters, try to turn Johan himself into another ghost.