The serendipitous ‘Letters of Note’ blog publishes ‘correspondence deserving of a wider audience’. It introduced a C S Lewis letter withe the comment that “what’s admirable is that he attempted to reply to each and every one of those pieces of fan mail, and not just with a generic, impersonal line “. So too did Rosemary Sutcliff, although I only have a couple of examples. (There must be several thousand out in the world in draws and treasure boxes). Lewis’s advice to a Narnia fan about writing was:

What really matters is:

1. Always try to use the language so as to make quite clear what you mean and make sure your 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.

Source: Letters of Note: C. S. Lewis on Writing.