A previously unknown letter from CS Lewis has been found in a secondhand book.

In the letter, dated 19 August 1945, Lewis writes to a Mrs Ellis about the real nature of joy, a subject he went on to discuss at length in his memoir 'Surprised by Joy: The Shape of my Early Life', which he started writing in 1948 and published in 1955.

"Real joy seems to me almost as unlike security or prosperity as it is unlike agony," Lewis writes.

"It jumps under one's ribs and tickles down one's back and makes one forget meals and keeps one (delightedly) sleepless o' nights. It shocks one awake when the other puts one to sleep. My private table is one second of joy is worth 12 hours of Pleasure. I think you really quite agree with me. (NB The physical sensations of joy and misery are in my case identical. Just the same thing happens inside me on getting the good or the bad news)."

The letter was found in a copy of A Problem of Pain, which the owner bought some years ago in a secondhand bookshop, but only recently discovered the letter.

Dominic Winter Auctioneers, who are managing the sale of the letter, believe it is previously unpublished. The letter is going up for auction on 18 December, with a guide price of £1,200 to £1,500.

Chris Albury, from the Gloucestershire-based auction house, told the Guardian: "We've only had a handful of good CS Lewis letters before and they've all attracted strong interest."

It is not yet clear who Mrs Ellis was, but the letter "wasn't a one-off correspondence," said Albury. "This is someone he knew personally. It's intimate, full of quite deep, philosophical thoughts, that you would only share with someone on the same wavelength."

Joy was a subject Lewis returned to repeatedly in his writing. In 'Surprised by Joy' he says: "The very nature of Joy makes nonsense of our common distinction between having and wanting."

In a postscript to the letter, Lewis writes: "Don't you know the disappointment when you expected joy from a piece of music and get only pleasure: Like finding Leah when you thought you'd married Rachel!"

The letter will be auctioned in a collection of children's and illustrated books, together with a letter by JK Rowling.