Crimson Joy by Robert B. Parker

A serial killer is on the loose in Beantown and the cops can't catch him. But when the killer leaves his red rose calling card for Spenser's own Susan Silverman, he gets all the attention that Spenser and Hawk can give. Spenser plays against time while he tracks the Red Rose killer from Boston's Combat Zone to the suburbs. His trap is both daring and brave, and gives the story a satisfying climax.