This sounds very interesting. I wrote on secular and Christian justifications for the modern notion of human rights in Looking for Bedrock: Accounting for Human Rights in Classical Liberalism, Modern Secularism, and the Christian Tradition (download here). Although I wasn't focusing on the history of human rights so much as what accounts for them, I looked into the notion of rights inherent in the Hebrew Scriptures and as developed by Christians in the early modern period. (For some of my blog posts on the history and justification for human rights try here, here, and here.)
I've asked my library to order "Christianity and Freedom." If Harper is right, the distinctively Christian contribution of human rights has deeper roots than I appreciated.