An excellent audio introductory Biblically-based critique of relativism by Blake LaMunyon, a pastor at Cherry Hills Community Church, is available here from his church as part of a series on "isms." It's also linked to this Apologetics 315 page. As part of his introduction, Blake recommends several basic texts that address relativism:

Blake's talk draws from the Great Banquet parable of Jesus at Luke 14:12-24 as he considers the Biblical articulation of absolute truth in view of John 14:1-6. By comparison, relativists call any absolute truth "narrow." Still, there are some universally accepted signals of a moral code (e.g., no one believes it is ethically acceptible to kick an old lady down the stairs). In both comparison and contrast, the Great Banquet parable emphasizes, on one hand, the absolute necessity of making the radical choice to answer the call to be a follower Christ in order to be invited to His banquet, and, on the other hand, the universal invitation to do so.