''Perdido, '' a fragment from a never completed longer work, is a rare and unexpected gift for Peter Straub's legion of fans. Even in this fragmentary form, it offers the sort of vivid, unexpected pleasures that only the finest imaginative fiction can provide. On one level, ''Perdido'' tells the story of a troubled family: a discontented husband and wife and the teenaged son who was--but is no longer--a musical prodigy. On another, it is the story of the isolated Norwegian resort known as Perdido, and of the impossible, dreamlike things that happen there. Perdido is a place where the rules of ordinary life no longer apply, where reality is malleable and infinitely strange. It is a place where ''you get what you didn't know you wanted'' and where lives are altered forever. For the unhappy couple invited to attend--and for the teenaged son awaiting their return--it is the place where a marriage ends and a life filled with alternate possibilities begins. Mysterious, evocative, and always superbly written, ''Perdido'' offers readers something genuinely special: a visit to an imaginary landscape that only Peter Straub could have created.