be set in 以...为背景 这个结构本来就是被动的.课文的原文是：He is best known for his novels set in his boyhood world on the river.这里是不是省略了which were?你可以看成是省略掉which were但实际上这句话中的set in 是非谓语动词中的过去分词 做定语。意思和which were set in..一样，语法不一样