The letter, now on loan from the Bancroft, goes on display at the Van Gogh Museum in Amsterdam this Friday as part of an exhibit titled "On the Verge of Insanity".
The letter and sketch also clear up how the ear came off and confirms that the injury was self-inflicted, which means that Gaugin had nothing to do with Van Gogh's missing ear.
Library assistant David Kessler searched through an uncataloged box devoted to Lust for Life, and finally retrieved a signed letter from Van Gogh's doctor with a sketch of his left ear detailing the self-mutilation., which attested to the fact that the artist removed all but the earlobe.
"Van Gogh's Ear: The True Story", authored by UK-born writer Bernadette Murphy, "reveals previously unpublished evidence about the painter's notorious breakdown", publishers Penguin India said.
The findings reflect on the infamous incident, which occurred in December 1888 in the southern French town of Arles, and was documented by newspaper reports and witnesses including police and fellow artists.
"Rey's letter includes drawings showing that Van Gogh cut off the whole of his left ear and not, as was long believed, just part of it", the museum said.
In order to pay off medical bills, Murphy found, she took a job as a cleaner at the brothel, and is likely to have been well known to Van Gogh in the small community.
He spent a year there, in which time painting became his remedy, but four months after leaving the institution he took his own life.
"She [Gabrielle] had a very nasty scar on her arm following the bite".
As part of the research, the author examined the lives of all the local prostitutes in Arles to uncover the story of this "mysterious girl, who was not a prostitute as has always been assumed, suggesting why the painter chose to give her his gruesome gift". Murphy traced the document to an American archive after finding a reference to it in a letter at a Dutch archive.
The exhibition On the Verge of Insanity focuses specifically on Van Gogh and his mental condition.
The show includes several masterpieces on loan from museums overseas, mostly significantly the 1889 oil on canvas, Portrait of Doctor Félix Rey, which is on loan from the Pushkin Museum in Moscow for the first time. Experts will argue his artistic works of genius were not the result of his illness, which one doctor diagnosed as epilepsy. There is no absolute proof it is the gun used by Van Gogh but the arguments, say experts are convincing.
After moving from the United Kingdom to Provence, France, she began researching the artist before presenting her findings to experts at the Van Gogh Museum in Amsterdam.