David C. Downing has written a post on how C. S. Lewis was often a mentor by mail. An excerpt:
Lewis’s advice to his correspondents often took the form of quotable epigrams. To a new wife who felt guilty over her mixed emotions about pregnancy, Lewis observed about guilt feelings, “You can’t help their knocking on the door; but you mustn’t ask them in to lunch” (3, 310). To a mother who asked Lewis to write a letter to her troubled daughter, Lewis answered prudently, “I think advice is best kept till it is asked for” (3, 320). On the same subject to the same correspondent, Lewis observed in another letter, “If few can give good advice, fewer still can hear with patience advice either good or bad” (369).