Thomas, famed for the radio play Under Milk Wood, died in 1953 at the age of 39. Officially the cause of his death in a New York hospital was "acute alcoholic poisoning" after a bout in which Thomas was said to have drunk 18 straight Bourbon whiskies.
But the book, The Death of Dylan Thomas by British biographer George Tremlett and North Carolina neurosurgeon James Nashold, will claim that Thomas was never as big a drinker as he was reputed to be.
The real cause of death, it will say, was that his American physician, Milton Feltenstein, mistook a diabetic coma for a drunken stupor and wrongly prescribed a course of injections including cortisone, morphine and benzedrine.
Tremlett was not immediately available for comment at the bookshop he helps to run in Laugharne, the Welsh village where Thomas once lived, and from which he got many of his ideas for the mythical town of Llareggub where the action in Under Milk Wood takes place.
Thomas, perennially struggling to live off his earnings as a writer, had gone to the United States in 1953 for a series of poetry recitals. His reputation, and his haunting deep bass voice, attracted crowds of fans to the events.
But on 4 November, his American mistress Liz Reitell called Dr Feltenstein to Thomas's room at the Chelsea Hotel in New York where he began the injections. Five days later Thomas died in the city, at St Vincent's Hospital.