The Fifth Heart by Dan Simmons

A dual-timeline mystery that intertwines a late-period investigation by an aging Sherlock Holmes with a modern scholar’s probe into a string of brutal murders and an unsettling legacy from the 19th century; as both narratives peel back layers of secrecy, they reveal a dark, supernatural conspiracy and long-buried betrayals, blending forensic deduction, gothic horror, and literary pastiche into a single, tense thriller.