Scottish crime scribe McDermid's latest is an absorbing new addition to the growing childhood-friends-are-party-to-a-murder-and-haunted-by-it-forever genre. Four tight-knit lads happen upon a bloody corpse one boozy, snowy night. They become suspects; they're never charged, but never cleared, either. And 25 years later, someone seeks revenge. ''Echo'''s first third grippingly tracks the psyche-shaping consequences of the crime among a host of characters. But what begins as a literary thriller -- an English ''Mystic River'' -- settles into a mere whodunit, well-mounted but short on psychological richness and stuck with a disappointing denouement. McDermid plays a crafty hand of poker, but her final card will leave you thumbing through the deck, wondering if she cheated.