First of all, note that these are letters not to Joy Davidson, but to another American lady. She’s a Southerner, older and frequently ill, which prompts Lewis to say that, though he has been critical of Britain’s welfare state, he likes the fact that everyone there has free healthcare — though he recognizes it means many go to the doctor with silly complaints.
Anyway, the book shows Lewis to be a good man: a patient, generous, and diligent correspondent; patient also and determinedly upbeat with how own (and Joy’s) serious ailments, too; hardworking; an unhypocritical, prayerful Christian generally.
Not a lot of theological insight that you wouldn’t see elsewhere. One exception (103, emphasis in original): “Yes — it is sometimes hard to obey St. Paul’s ‘Rejoice.’ We must try to take life moment by moment. The actual present is usually pretty tolerable, I think, if only we refrain from adding to its burden that of the past and future. How right Our Lord is about ‘sufficient to the day.’ Do even pious people in their reverence for the more radiantly divine element in His sayings, sometimes attend too little to their sheer practical common-sense?”