Jo Nesb writes, in his introduction to the reprint of 1967's "The Man on the Balcony," that "artists stand on the shoulders of those who have come before."  Indeed, the husband and wife writing team of Maj Sjwall and Per Wahl, in their Martin Beck police mysteries, helped set the standard for crime fiction that is often imitated but rarely duplicated.  "The Man on the Balcony," skillfully translated from the Swedish by Alan Blair, is based on a real case that occurred in Stockholm in 1963 in which two small girls were abducted, sexually abused, and murdered.  This chilling story brings home the obstacles that law enforcement officers face when they attempt to identify and apprehend a suspect who lives beneath the radar and leaves few helpful clues.  Even as they sift through mountains of data and telephone tips, the jaded detectives fear that all their efforts may ultimately prove fruitless.