The Black Book (rebus #5), Ian Rankin

When a close colleague is brutally attacked, Inspector John Rebus is drawn into a case involving a hotel fire, an unidentified body, and a long forgotten night of terror and murder. Pursued by dangerous ghosts and tormented by the coded secrets of his colleague's notebook, Rebus must piece together the most complex and confusing of jigsaws. But not everyone wants the puzzle solved - perhaps not even Rebus himself...