Obsession, by Jonathan Kellerman
Teenager Tanya Bigelow, whom Delaware treated as a child for obsessive-compulsive disorder, consults him because her aunt Patty, who raised her, conveyed a cryptic message just before she died, apparently confessing to a crime. Shortly after Delaware and Sturgis start investigating, one of Patty's former neighbors turns up dead, the first in a series of corpses that appear, possibly as a result of the duo's turning over old rocks. Since the identity of the killer is revealed relatively early on, the final sections are short on suspense.
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