Dylan Thomas’s marriage was notoriously tempestuous, blighted by drunken brawls and mutual infidelity until the poet drank himself to death at the age of 39.

On his death bed, his wife, Caitlin, reportedly burst into his hospital room asking: “Is the bloody man dead yet?”

More than half a century later, however, a different side to the relationship is revealed in her private journal.

The yellowing leaves of a school exercise book are filled with poignant meditation on her love for the Welsh author of Under Milk Wood – his play about an imaginary village called Llareggub (“bugger all” backwards) – and his poetic reflection on death, Do Not Go Gentle into That Good Night.

Letters from the heart of an incomparable wordsmith

Dylan Thomas's love letters to his wife, written by hand with stubby pencils on any scrap of paper

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• Secret diary reveals wife's love for Dylan Thomas

Two years after Thomas died in 1953, Caitlin was pining for a man she remembered as “sweet”. Her journal counters the image of Thomas as a womanising drunk, which has endured since John Malcolm Brinnin’s 1955 biography Dylan Thomas in America, which Caitlin attacked as a “betrayal”. Imagining him in his grave, she writes: “Oh God, oh Dylan, it must be cold down there; it is cold enough on top, in November: the dirtiest month of the year that killed you on the ninth vile day. If only I could take you a bowl of your bread, and milk, and salt, that you always drank at night, to warm you up.

“I am not going into that waste allotment of a T. S. Eliot elegy of a cemetery. Dylan will have to move up, in his single ditch, snug under the cliff, and make room for me; then we can keep each other warm, or cold, or maggot breeding.”

In another passage, she mused: “And sometimes I have the nerve to pretend that this Dylan love never was. I can make myself believe in the superfluity of this love for quite a long time; then it catches up with me, and it is all the crosser for being ignored.”

The journal is among 40 manuscripts and inscribed first editions amassed by a New York enthusiast. The collection is being sold by Rick Gekoski, a leading London dealer in rare books, and is valued at £250,000.

George Tremlett, a biographer of Thomas and Caitlin, said that the journal could help to reverse the damage done by Brinnin’s biography and, more recently, by the filmThe Edge of Love, starring Matthew Rhys, Sienna Miller and Keira Knightley. Brinnin’s depiction of Thomas as an alcoholic was wholly false, Tremlett said, but it had “set the mood, defining how people have thought of Thomas”.

Pubs were an important part of Thomas’s life, he acknowledged. But, while the writer could hold a pub audience spellbound with jokes and extempore poems, he could not hold his drink beyond two pints, he believes. While there were affairs, he added, the evidence is fewer than some suggest.

Caitlin Macnamara, who died in 1994, was Thomas’s muse. The free-spirited daughter of a bohemian Irish family, she might have pursued a career as a dancer had she not married Thomas in 1937. Although she co-wrote a memoir with Tremlett in response to Brinnin’s biography, she regretted not doing more to emphasise her relationship’s tenderness. Dr Gekoski said: “This is her first effusion about Dylan. It’s so important . . . and so impassioned.”

Another item up for sale, Thomas’s first book, Eighteen Poems (1934), bears a touching inscription: “From Dylan to Caitlin – lovingly in spite”. The collection also includes letters. In 1945 he wrote to Caitlin worrying about their finances before signing off: “I shall be with you, my true love until death & forever afterwards.”

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