THE BURNING QUESTION in Riviera Blues (Macmillan, 271 pages, $19.95 cloth) is not "Whodunit?" but "Who is it written for?" Could Canada possibly be producing a writer who aims his novels at the denizens of just one city, even if that city is (gasp) Toronto? Or is it possible that to the much wider reading public, Toronto has become as authentically individual -- as mythic, even -- as London, Paris, Berlin, New York, or LA? If the answer to either of these questions is "yes," then Jack Batten and his publishers yes, know exactly what they`re doing. Which is producing mystery novels in which the sleuth- hero`s lifestyle takes precedence over the plot. In Riviera Blues, Crang combines a holiday in France (where Annie, his moviereviewer lover, has been assigned to the festival at Cannes) with investigating a Toronto trust company employee who`s enjoying a suspiciously extravagant vacation in Monaco. Several hundred wisecracks and injokes later, Crang`s enquiries threaten to get serious, but basically Batten approaches his story like P. G. Wodehouse impersonating Dashiell Hammett. It`s mostly good fun, as long as Toronto and seen-through-Toronto-eyes lifestyles fascinate you and serious mystery novels don`t.