I never read much of Lewis - tried to go through the Screwtape Letters and it was as torturous as The Book of Numbers in the Bible to me. Seemed he did some self-indulged working things out in his mind and published the journey rather than the wrapped up gist - of course, when my mind gets bored, I can miss a lot of the important stuff.
Screwtape Letters was a fantasization of the affairs of hell. Its attraction would be in the intrigue. If it looks to you like Numbers then maybe you aren’t of a particular bent to appreciate it.
This might not be the most logical Lewis since it is a fantasy genre and its truths, at best, are metaphorical.
Mere Christianity is an attempt to argue towards Christianity, but it can’t foresee every proposed counter argument. The modern apologist begins where C. S. Lewis left off.