From a 1976 interview of Aurelia Plath by Robert Roberton, published in The Listener, Vol. 95, p. 516. Aurelia had just described Sylvia Plath's sequel to The Bell Jar, an autobiographical novel "provisionally titled The Hill of Leopards," about an American Fulbright student who marries and has her first child in England.
Roberton: And yet I believe that you saw this manuscript destroyed.
Aurelia Plath: Yes, this was one of the most terrible experiences of my life, really. She had built an enormous bonfire in the court outside her home in Devon. I stood in the doorway, holding her little daughter by the hand, and holding the boy, her son, in my arms, not able to go to her. And then I saw her emerge from the household with her arms full of manuscripts, and I saw the second volume, in rough draft, which was to be a gift to, and a surprise, to her husband -- she tore the pages apart bit by bit and fed them into the flames. She fed much else into the flames.
Roberton: There was no way of stopping her?
Aurelia Plath: No, I couldn't. I couldn't leave the children.
The above, which supposedly happened on Aurelia's visit to Court Green in summer 1962, is probably false. In an unpublished version of the introduction to Letters Home, Aurelia describes in elaborate detail Sylvia sitting her down earlier on that visit and reading to her from this manuscript, written supposedly in honor of and dedicated "To Ponter" (Sylvia's private nickname for her husband Ted Hughes).
This assumes that Sylvia had labored over a lengthy creative work she intended as a gift, but Sylvia wrote for money and never wasted any of her precious writing time on giveaways. She had already dedicated The Colossus to Ted. Sylvia left no trace of this supposed second "happy" novel: no rough drafts, no calendar notations, no mentions in letters to friends: nothing.
Aurelia Plath wanted readers to believe that Sylvia after writing The Bell Jar -- a mean-girl novel which harmed Aurelia's reputation and the other real people it caricatured -- wrote an anti-Bell Jar, a joyous novel about an Esther-Greenwood-type character in love, married, and having a baby, that unfortunately Sylvia burned after showing it only to Aurelia.
While writing The Bell Jar in 1961, and preparing it for publication during 1962, Sylvia wrote notes in her calendar and letters to her brother, her agent, friends, and publishers -- keeping the book secret only from her mother. Of the "joyous" "second novel" Sylvia made no mention. Aurelia's editor rightly deleted from the draft of Letters Home Aurelia's "she read to me from her joyous second novel" scene. The anecdote as written rang false and could not be verified. Aurelia did not write down what Sylvia had read, but she wrote that Sylvia had said that art and life were not the same, something Aurelia dearly wished her daughter would have said.
After confirming Ted's adultery in July 1962, Sylvia did begin an angry novel about a cheating husband titled The Interminable Loaf and then renamed Double Exposure. That manuscript actually existed, because Sylvia mentioned it in letters and on her calendar, and Ted and Olwyn Hughes and Assia Wevill read the unfinished 60 or 70 pages after Plath died. All were horrified by Plath's savage caricatures of themselves. Gosh, when they received The Bell Jar treatment, just like Aurelia they were shocked and didn't like it! That manuscript is considered "lost" and rumor says it might one day be found.
Biographers have guessed that Sylvia also burned all her mother's letters in that bonfire of July 1962 or another bonfire soon after, but Aurelia did not say that in any letters or interviews I have read. I think Aurelia, who survived her daughter by 31 years, would have mentioned somewhere in her many hundreds of letters to others, or in Letters Home, how much Sylvia's burning of her letters hurt or disappointed her. In fact after some thought I think Sylvia didn't burn them at all.