Джек Вэнс

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 …

The Man in the Cage is a book by Jack Vance.