With the publication of famous Leather-stocking Tales, the first of which were The Pioneer, the second The Last of the Mohicans, Cooper established his reputation as the first major American novelist.

【分析】这里的the first of which were The Pioneer, the second The Last of the Mohicans 应该是the first of which was The Pioneer, the second of which was The Last of the Mohicans 的省略形式（包含有两个非限制性定语从句）。所以，原句中的 were 应该改为 was 。