Jack Vance 
Moon Moth and Other Stories [EPUB ebook] 

When an out-worlder’s body was found in the river, Thissel knew where the criminal was – behind one of the masks worn by the remaining out-worlders on Sirene. But how could he tell which one, on a world where everybody lived behind masks, where men never spoke but sang to instruments, and where any act of intervention with another man’s business was punishable by death!