On the planet Ardagh 1, every human colonist soon acquires a symbiotic bond with an alien that takes the form and personality of a human who has died. The alien “ghost” feels terrible pain if kept from its human counterpart, but interacting with mimicked loved ones and even relative strangers is so emotionally costly to the humans that Dr. Grayson Murphy institutes the Ghost Protocol, which instructs humans to completely ignore their ghosts. New arrival Elizabeth Cole, a psychologist in training, is very interested in the ghost/human bond—all the more so when she realizes that she died en route to Ardagh 1 and has been reincarnated as Grayson’s ghost. She tries to continue her research while being compelled to stay in Grayson’s presence at all times. This becomes all the more awkward when attraction develops between them, since Grayson could lose his career if he so much as acknowledges Elizabeth’s existence. The far-fetched premise holds up surprisingly well in an absorbing and exciting story full of science, sex, and intriguing plot twists. (Nov.)