I think
The Badger might approve of this one (and I do have to confess to being influenced by him in choosing to pursue more Lewis reading).
I hesitated to include it here because this is ostensibly a list of works of fiction. But what is a mild dilemma for me is, fully and truly, the genius of Lewis. His choice to write about the Christian life and, in particular, the struggle of a Christian to overcome temptation and to be, with Paul, wise to the schemes of Satan, by means of letters from a senior devil to his nephew makes his work rise to a greater height than any straightforward work of theology could have attained.
I've quoted on this blog from it before, here & there. Those excerpts - and they could be multiplied many times over - show an acuteness of insight that regularly leaves me speechless with admiration and sombre in reflection. I cannot recommend it highly enough.
And if The Caped Marauder (sorry, The Masked Badger) and I ever get around to posting a list of theological works, I daresay it will appear there too.
Deservedly.