In a literary version of Cash in the Attic, a rare proof edition of Sylvia Plath’s The Bell Jar which has sat on a spare room bookshelf for the last quarter of a century, is due to be auctioned in London next month.

The uncorrected proof copy of Plath’s only novel, a semi-autobiographical account of a young woman’s spiral into depression in New York, states that it is “not for sale”. The 1962 proof is attributed to “Victoria Lucas”, the pseudonym under which The Bell Jar was published in 1963, shortly before Plath’s suicide aged 30. It would not appear under Plath’s own name until 1966.

Bonhams, which will auction the proof on 24 June, says there are more than 70 textual variations between the 1962 uncorrected proof, and the final published first edition from Heinemann in 1963. In the proof, the name of the novel’s heroine remains “Miss Lucas” on two pages – the pseudonym under which Plath published the novel, rather than the character name Esther Greenwood.

The name substitution comes when Plath’s heroine is in hospital, having attempted to kill herself by swallowing handfuls of pills. Instead of being asked “And how are you feeling this morning, Miss Greenwood?”, the proof reads “And how are you feeling this morning, Miss Lucas?” Plath continues: “I … hate people to ask cheerfully how you are when they know you’re feeling like hell and expect you to say ‘Fine.’ ‘I feel lousy.’”

The Bell Jar proof copy coming to auction. Photograph: Bonhams/PR

The proof also shows how Plath switched the name of one of her characters. “I collected men with interesting names,” she writes in the final edition. “I already knew a Socrates. He was tall and ugly and intellectual and the son of some big Greek movie producer in Hollywood, but also a Catholic, which ruined it for both of us.” The character was originally named Plato, the proof shows.

“These are very rare,” said Matthew Haley, the head of the book department at Bonhams. “I hadn’t seen one until we sold one in 2013 for £5,000. There aren’t many around – I’d say in the dozens rather than in the hundreds.”

Bonhams quotes Plath expert Peter K Steinberg on the changes in the text. He says that the differences “are the result of edits made either by Plath herself when she reviewed the proof or by the editors as they prepared the final typesetting” and show “that Plath read her proofs of The Bell Jar very carefully”. The changes extend “our understanding of her involvement in the creative process beyond the composition of the work itself”, according to Steinberg.

The edition had previously been sitting in the spare room of an ex-civil servant in Wiltshire for the last 25 years. The woman, who blogs as Lucy WithaY, said she bought it in a Winchester secondhand bookshop during her first year in college in 1985, as it was required reading.

“I remember buying it, but not how much it cost. It can’t have been much, because I never had any money as a student,” she told the Guardian. “I asked the chap in the shop what the ‘not for sale’ note meant on the front as I was slightly anxious I might be buying something a bit dodgy. Anyway, I took it to lectures throughout that term, used it for essays and so on, and treated it just like all my other books, except that I never wrote my name in it.”

A few weeks ago, she was looking at eBay and the book “popped into” her head, so she did a search. “Nothing on eBay, so I googled it, and found the website that talked about how rare it is, and then the link to the Bonhams page,” she said. “Very exciting.”

She got in touch with Bonhams, which asked her to send it in. “It’s relatively unusual for things to come to us from people who don’t know what they’ve got – mostly people do, and they’ve inherited them, or bought them deliberately,” said Haley. “To end up with it by accident is rather fun.”

Bonhams has set a guide price on the edition of £2,000 to £3,000; Lucy WithaY writes on her blog that she is hoping to use the proceeds to fund a holiday to Japan.