I saw this on the excellent Brandywine Books. It's from The Collected Letters of C.S. Lewis, in which Lewis writes to his brother Warren on July 20, 1940:
Before the service was over – one cd. wish these things came more seasonably – I was struck by an idea for a book wh. I think might be both useful and entertaining. It wd. be called As one Devil to Another and would consist of letters from an elderly retired devil to a young devil who has just started work on his first ˜patient'….
Awesome!
I also think it's cool that he got this inspiration in church.
In that same collection he also refers to "Till We Have Faces" when it was still just a silly idea.