Kay Scarpetta, Cornwell's heroic blond forensic pathologist, faces another rendezvous with the vile Chandonne twins, whom she failed to vanquish in two previous novels. The hideously deformed Jean-Baptiste (he once tried to hack Scarpetta with a chipping hammer) is safely moldering in prison. But his brother Jay (he tried to torture her with a heat gun) eluded authorities and is hiding out in the Louisiana bayou with a gal pal who's been kidnapping Scarpetta look-alikes for Jay to dismember. The material is outrageously campy, and ''Blow Fly'''s second half suffers from pretty severe plot decomposition. But Cornwell writes, as usual, with unwavering intensity in this grisly, fast-paced thriller. The effect: utterly chilling.