Silent Prey (Lucas Davenport #4) by John Sandford

He was right. His guards slain, the brilliant, insane pathologist of Eyes of Prey flees to New York, there to continue his research into aspects of death. Carefully, he conducts his experiments, searching the eyes of his dying victims for what they can reveal, the mounting body count causing an uproar in the city.

In desperation, the police reach out for the man who knew Bekker best, but when Lucas arrives, he finds unexpected danger as well. For Lily Rothenburg, the policewoman whose intense affair with Lucas has never completely faded, is there too. Now, consumed with her own investigation of a group of rogue killers within the police department, she draws Lucas into her orbit again, until their hunts merge, their twin obsessions driving them ever closer to the edge . . . and then over.