Jenni and I recently read this book:

C. S. Lewis. C. S. Lewis Letters to Children.  Edited by Lyle W. Dorsett and Marjorie Lamp Mead. New York: Macmillan, 1985. 120 pp.

Tim and Kathy Keller mention it in The Meaning of Marriage:

As a girl of twelve, Kathy wrote to C. S. Lewis and received answers from him, which she taped to the inside covers of her copies of the Narnia Chronicles. His four letters to her (to “Kathy Kristy”) can be found in his Letters to Children and the third volume of Letters of C. S. Lewis. (p. 245, note 2)

C. S. Lewis wrote his third and fourth letters to Kathy less than a month before he died.

Lewis’s letters are fun and instructive to read. They are filled with his typical wit, and Lewis models how adults should treat children with respect.

11 excerpts:

1. [O]ld people can be quite as shy with young people as young people can be with old. This explains what must seem to you the idiotic way in which so many grown-ups talk to you. (p. 25)
2. Remember that there are only three kinds of things anyone need ever do. (1) Things we ought to do (2) Things we’ve got to do (3) Things we like doing. I say this because some people seem to spend so much of their time doing things for none of the three reasons, things like reading books they don’t like because other people read them. (p. 27)
3. [Aslan] is the Turkish for Lion. I pronounce it Ass-lan myself. (p. 29)
4. I don’t think age matters so much as people think. Parts of me are still 12 and I think other parts were already 50 when I was 12: so I don’t feel it v.[ery] odd that they grow up in Narnia while they are children in England. (p. 34)
5. I don’t believe anyone is “good” or “bad” at languages. If you ever want really badly to read something which you can’t get in English, you’ll find you can learn a foreign language alright. (p. 37)
6. I am so busy marking examination papers that I can hardly breathe! The very good ones and the very bad ones are no trouble, but the in-between ones take ages. (p. 47)
7. About amn’t I, aren’t I, and am I not, of course there are no right and wrong answers about language in the sense in which there are right and wrong answers in Arithmetic. “Good English” is whatever educated people talk; so that what is good in one place or time w[oul]d. not be so in another. (p. 63)
8. What really matters is:—
   1. Always try to use the language so as to make quite clear what you mean and make sure y[ou]r. sentence couldn’t mean anything else.
   2. Always prefer the plain direct word to the long, vague one. Don’t implement promises, but keep them.
   3. Never use abstract nouns when concrete ones will do. If you mean “More people died” don’t say “Mortality rose.”
   4. In writing. Don’t use adjectives which merely tell us how you want us to feel about the thing you are describing. I mean, instead of telling us a thing was “terrible,” describe it so that we’ll be terrified. Don’t say it was “delightful”; make us say “delightful” when we’ve read the description. You see, all those words (horrifying, wonderful, hideous, exquisite) are only like saying to your readers “Please will you do my job for me.”
   5. Don’t use words too big for the subject. Don’t say “infinitely” when you mean “very”; otherwise you’ll have no word left when you want to talk about something really infinite. (p. 64)
9. I think I agree with your order for reading the books more than with your mother’s. [Editor’s note: Laurence’s mother felt that the seven Chronicles of Narnia should be read in the order in which they were published, since she assumed that this sequence was intentional. Laurence, however, believed that the stories should be read chronologically according to Narnian time . . . .] The series was not planned beforehand as she thinks. . . . So perhaps it does not matter very much in which order anyone reads them. I’m not even sure that all the others were written in the same order in which they were published. (p. 68)
10. I am so glad you like the Narnian stories and it was nice of you to write and tell me. . . . Do you know Tolkien’s The Lord of the Rings? I think you w[oul]d. like it. (p. 75)
11. American university teachers have told me that most of their freshmen come from schools where the standard was far too low and therefore think themselves far better than they really are. This means that they lose heart (and their tempers too) when told, as they have to be told, their real level. (p. 84)