A rare and important letter written by Charlotte Brontë in between the deaths of her brother and a sister is expected to fetch up to $100,000 at an auction in New York today.

The letter, dated October 18, 1848, is to her publisher William Smith Williams, of Smith, Elder & Co, and will be up for auction at Sotheby’s in New York, with a reserve of $70,000 to $100,000 (£50,000 to £71,000).

The letter is one of two up for auction from the James S Copley Library, based in California.

The second, dated April 3, 1850, appears to give medical advice to her lifelong friend and correspondent Ellen Nussey. It has a reserve of $30,000 to $50,000 (£21,000 to £35,000).

Justin Caldwell, vice-president of the department of books and manuscripts at Sotheby’s, New York, said the first letter was of particular interest.

He said: “This is when she is working on her second novel Shirley. She’s had a big success with Jane Eyre of course and I would have to say this letter is certainly written on a day when she is very down.

“It’s certainly a letter full of gloom, I have to say. It is also an interesting letter because she is in mourning.”

The letter was written in the brief time between the deaths of her brother Branwell on September 24 and her sister Emily on December 19. Mr Caldwell said the second letter was also of interest and both had attracted interest on either side of the Atlantic.

He said: “I would say that certainly all three of the Brontës have a wide following here as well as in the UK, so I would expect some American bids, from other American institutions as well as collectors.

“So I would say that certainly for these two letters there will be trans-Atlantic competition. I’m really glad we have got these letters, we don’t see them too often here.

“These have been hidden away in the Copley Library for many years so they are fairly fresh on the market and with interesting content.”