I will do it for him.
The first and best reason not to write about C. S. Lewis is because he is almost always more clear and more insightful than his would-be expositors. Lewis is a model of clarity and concision. He is both more perspicuous and more brief than those who mediate his thought. When you compare their work to his, the effect is like trying to illuminate the sun with a flashlight. We see his interpreters better by his light than we see him by theirs. No doubt they have seen this phenomenon at work in others. How they think they themselves managed to avoid that fate is never articulated.
The second reason why we generally ought not write about Lewis is that the scope of his wisdom and of his corpus is too wide for most scholars to master and to therefore to tie together. To mediate Lewis’s expansive contribution requires of us something like what it required of Lewis: impressive skill in (1) Medieval and Renaissance language and literature, in both its Continental and its British embodiments; (2) the history of western philosophy from the ancient Greeks onward; (3) the history of western theology from the ancient Jews onward; (4) the history of science fiction; (5) the history of literary criticism from the ancient Greeks onward; (6) the history of the novel, both Continental and British; (7) Norse (and Icelandic) language and literature; and (8) the history of western spirituality. The list is shockingly partial. Lewis also was a practicing novelist, a poet, a critic, and a professor. His explainers, on the whole, are not. When they try to tie his though together for readers even more narrowly adept than they are, his interpreters are out of their depth in so many fields all at once
I know of only a few notable exceptions. Here I name both Donald T. Williams and Michael Ward. Williams can write fine books and articles about Lewis and Co. simply because he lives in the world they used to inhabit. He too is a dinosaur of sorts, a survivor from another age, born out of time. He too is a poet, a philosopher, a storyteller, a critic, a theologian, a teacher and, above all, a reader. Ward succeeds because he is not simply a synthesizer but has a point to argue. He has a thesis to prove, and he proves it. He writes more than mere summary or comment. He makes an argument, a case. He drives you back to Lewis himself in order to find out if Ward is correct. (He is.) Most of Lewis’ modern interpreters drive you away from Lewis; they hinder encounters between you and him, although nothing is normally further from their intention.
And then of course, there is Walter Hooper – indefatigable, insightful, and generous, a living encyclopedia of Lewisiana. If Hooper had done nothing else his entire professional life but edit the three enormous volumes of Lewis’ letters, his contribution would tower. If you have a question about Lewis, any question, go first to Hooper to see if he has written on it.
But for the special few exceptions, I think Gordon Saunders was quite right. In most cases, if you can avoid writing about Lewis, you should.
I now think the same holds true for writing about John Milton and Ronald Reagan.