The abandoned work he referenced to Henry is likely Letters to Malcolm: Chiefly on Prayer, published posthumously in 1964. It didn't come together until it was set in the context of an imaginary conversation with a fictional friend. It also appears that Lewis opted for a less straightforward apologetic approach following a debate with female philosopher G. E. M. Anscombe at the Socratic Club on the topic of miracles, a debate after which some felt Anscombe was the clear winner. And there are other examples in his public addresses and personal correspondence where Lewis explained with transparency how defending the gospel had taken its toll.
Ultimately, the fall of the Third Reich brought with it an end to Lewis's direct apologetic. And though Britain was at peace, Lewis continued to fight another battle until his death in 1963. Like the deep magic of Narnia, this battle was not with flesh and blood but with powers and principalities. From wartime talks to talking fauns, his excellent life was committed to the advancement of the gospel. Though dead, yet still he speaks.
Dan DeWitt is the dean of Boyce College, the undergraduate school of The Southern Baptist Theological Seminary in Louisville, Kentucky. He blogs at Theolatte.com.