J.G. Zymbalist's debut is a quirky but poignant coming-of-age tale about children, Martians, freaky Martian hummingbird moths, and alluring sea nymphs.     The first thread relates the suspenseful tale of a Martian girl, Emmylou, stranded in Maine where she is relentlessly pursued by the Pinkerton Detective Agency's Extraterrestrial-Enigma Service.  The second thread concerns her favorite Earthling comic-book artist, Giacomo Venable, and all his misadventures and failed romances.  The final thread deals with a tragic young lad, Rory Slocum, who, like Emmylou, loves Giacomo's comic books and sees them as a refuge from the sea nymphs or Oceanides incessantly taunting and tormenting him.     As much as anything, the triple narrative serves to show how art may bring together disparate pariahs and misfits--and give them a fulcrum for friendship and sense of communal belonging in a cruel world.