The collected letters of CS Lewis, edited by Walter Cooper, are in three volumes - and the first two volumes - which span from Lewis's childhood to 1949 are the best. After Lewis became famous; most of his correspondence was with people he did not know well (or at all); and these later letters are therefore mostly more public and generic in tone.
In the early letters you will find more of Jack Lewis's humour than in any other of his writings - displayed in the letters to his brother Warnie (then away as a Captain in the army, sometimes abroad), his childhood friend Arthur Greaves, and to other friends from college days.
Indeed, it is in these letters that I find the most vivid impression of Lewis as a man - far more so than in any of his published writings. I get a distinct flavour of that man who formed and sustained The Inklings, who was (legendarily) the life and soul of the company - as well as loyal and considerate.
I find 'Lewis the letter writer' to be a very likeable persona; more likeable than Lewis the theologian, literary critic or adult novelist - and even-more likeable than the author of Narnia
I don't know whether others will tune-into Lewis's humour in the way that I do (humour is very much a matter of taste); but I find it to be an absolute delight. In the first place - there are many parts which are laugh-out-loud funny; which is very rare among published private letters (Kingsley Amis's correspondence with Philip Larkin is another example: these letters are very funny indeed, albeit in a characteristically dark and cruel way!).
Yet Lewis's humour is also warm and affectionate, probably because of the affection between Lewis and his correspondent - it is a generous humour; shared in order to give enjoyment to the recipient - and tailored to each recipient.
So Lewis's funny passages sent to Warnie are different from those for Arthur Greaves, and these again from Owen Barfield and so forth. Lewis is sharing running jokes and the 'kind of thing' that provides amusement in a particular friendship; the 'kind of thing' that particular friends talk-about.
The quality of writing is superb - the letters are as well-written as the published prose (although not so well spelled!) - which is unsurprising given that Lewis (like Samuel Johnson) composed in his ind, not on paper - and usually published his (corrected) first drafts.
The famously great powers of quotation that Lewis displayed in conversation are also on view - and it is made clear that these were based mainly on the capacity to pastiche rather than on an exact memory.
Lewis's 'quotes' are nearly all more-or-less inaccurate, at a word-by-word basis (as Walter Hooper's footnotes make clear), but always contain the flavour, distinctiveness and pertinence of that being quoted.
This is the same 'method' as deployed by a Scottish novelist I used to know - Alasdair Gray. He would fluently improvise 'quotations' from favourite works that cropped-up in conversation; getting the spirit exactly right while - strictly speaking - using mostly his own words. For example - he did this when we discussed funny parts of Kurt Vonnegut's Slaughterhouse Five; and again with Flann O'Brien's At Swin Two Birds - also done in an impression of the appropriate US or Irish accent. The business was both hilarious and virtuoso.
I know that however much I recommend Lewis's collected letters; very few people are likely to read them.
Very few people read 'secondary literature' (biography, letters, criticism) of even their very favourite authors.
But I believe that if you have not at least dipped-into CS Lewis's letters - there is also a 'selected letters' available, and the complete letters to Arthur Greaves are published separately as They Stand Together - then you don't really know him.
And, strictly, as writing; the best letters are Lewis at his best; equaled (in various different ways) but never surpassed by anything else he wrote.