Strongly agree about the excellence of Tolkien's letters. Indispensable.
C. S. Lewis's letters are superb. I have loved them ever since, as a youth, I failed in being good company for my cousins because I wanted to keep reading them. This was, I think, about 42 years ago, btw.
After Lewis became well-known as a BBC broadcaster on religious topics, he heard from many people with spiritual questions, intellectual conflicts about faith, and so on, and he replied; and perhaps some Chrons people wouldn't find these letters to their taste, but it would be a shame to miss many others that excel in
--accounts of people who met, or even got to know well. For a specimen, one might read his account of a visit to W. B. Yeats;
--lovely narratives of his walking tours;
--endless pages of good book-talk.
One might get hold of the letters to his friend Arthur Greeves, published as They Stand Together. They became friends as adolescents and kept in touch all their lives, i.e. something like 45-50 years. It is delightful to read Lewis bursting with enthusiasm over his latest discovery, in his teens, e.g. Sir Thomas Malory or Algernon Blackwood -- some of his discoveries coming thanks to Arthur. I don't know what Chrons people would expect C. S. Lewis's friend (other than his brother) of longest duration to be like. Arthur was artistic, semi-invalid, homosexually inclined, and eventually became a Baha'i, if I'm not mistaken.
Lewis's letters to his brother Warren ("Warnie") are also very good, with plenty of walk- and book-talk. I have the three thick volumes of Collected Letters and one can skip along, reading all the letters to a given correspondent.
There is a small book of the letters of Lewis and Arthur C. Clarke (From Narnia to a Space Odyssey). It is an abomination. The Clarke letters as printed therein are fine, as far as I know without having seen the originals, but the Lewis letters are so badly transcribed as to be nonsensical at times. Fortunately, Lewis's letters to Clarke appear in intelligible form in the Collected Letters. They didn't correspond a whole lot.
I liked Jackson's Oxford paperback of selected letters of Samuel Taylor Coleridge, especially five autobiographical letters, and Gittings's selection of the letters of John Keats.
A World's Classics paperback of letters of Charlotte Brontë was worth reading, but not very cheerful. The letters of William Cowper are said to be good, but I haven't read my copy.
I liked the letters of poet Ruth Pitter, but I don't know how much interest they would have to someone who hasn't read her poems, which I like very much. She knew Orwell and C. S. Lewis.
Someone is bound to recommend the copious letters of that prolific letter-writer H. P. Lovecraft, but I await a selection that will have more appeal to me than the books I've seen. At one time I had four of the five Arkham House volumes, but I gave them away to the university library years ago and haven't regretted the decision. His accounts of his penny-pinching road trips to New England antiquarian sites etc can be fun to read, but his expositions of a materialism rather vulnerable to critique are apt to be tedious, and the limitations of his sympathies are liable to become evident. I would have thought the correspondence of Rider Haggard and Rudyard Kipling would be a cinch to be good reading, but I ended up selling my copy, it seems, without having read it through -- anyway I don't have it now. I have read a volume of letters by Arthur Machen and found them quite disappointing!
I own the two volumes of the letters of my favorite artist, Samuel Palmer, but I haven't read them yet. I have that "life in letters" selection of Orwell's letters awaiting my attention. I must have read most of the letters in the four volumes of collected journalism etc that appeared in the 1970s. I have a 2-volume selection of Tolstoy's letters on hand, which I haven't read, although I liked the letter he wrote late in life in which he lists a bunch of things he read at various stage of his life that made an impression. The letters of Wendell Berry and Gary Snyder, Distant Neighbors, has been a little disappointing.