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.
Thanks - maybe I’ll give the Letters another try.