Here are the literary citations in Piper's book about Cowper (although I know not of their content):
- Gilbert Thomas, William Cowper and the Eighteenth Century (London: Ivor Nicholson and Watson, Ltd., 1935)
- George Melvyn Ella, William Cowper: Poet of Paradise (Durham, England: Evangelical Press, 1993)
- George Herbert, "The Pulley," in Eerdmans Book of Christian Poetry, comp. Pat Alexander (Grand Rapids, MI: William B. Eerdmans Publishing Co., 1981)
- William Cowper, The Poetical Works of William Cowper, William Michael Rossetti, ed. (London: William Collins, Sons and Co., n.d.)
And from what I gathered in the book, though I could be mistaken, there is a good amount of biographical information in Cowper's books of poetry, like The Poetical Works.