Why did Lewis tell Joan not to read Wordsworth’s prelude until she is older?
C. S. Lewis shared a correspondence with a young American fan of his Chronicles of Narnia series named Joan Lancaster. Being a young reader and writer, she was rather inexperienced but very energetic in her pursuit of literature and writing. She corresponded frequently with Lewis, and he confided in her the secrets of being a writer.
At one point, he mentions to her Wordsworth's Prelude, a particular piece of writing of which he is clearly very fond. When describing to Joan how she will struggle as a writer to find the perfect way to describe something, he advocates reading this piece—but says to do it when she comes around to the work herself in roughly ten years. Otherwise, he says, she'll spoil it for later reading.
Essentially, Lewis is telling Joan that this piece of writing is so wonderful and valuable to read that she shouldn't read it just yet. There is so much other literature that she won't be able to enjoy properly because she will be ruined by the beauty of Prelude, and therefore she should allow herself the time to enjoy those works. Additionally, she will likely feel insecure in her own writing, and so it would be more advisable to read it when she has become more accomplished.
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