Some of them look as if they have been written by a seven-year-old, although it would be an unusually developed child who wrote so gleefully about putrefying birds, Daumier lithographs and his joy at purchasing a zebra's head.
In fact the letters were written by Lucian Freud, Sigmund Freud's grandson, in his early twenties, and they are packed with his waspish wit, skittish restlessness and pleasure at the high-low life he lived so rapaciously.
This week Sotheby's will auction this previously unseen cache of letters, mostly written in 1943 and all of them addressed to Felicity Hellaby, one of Freud's first girlfriends, whom he met at art school.
Oliver Barker, Sotheby's senior international specialist in contemporary art, said that the letters provided fascinating insights into Freud's life, thoughts and interests.
"They kind of have everything," he said. "Paintings he was working on, plays that he'd seen, films he'd seen, jackets that he'd bought, props he'd bought, compositions he was struggling with."
Freud and Hellaby met at the East Anglian School of Painting and Drawing in Dedham, Essex. They were teenage sweethearts, although it was an unusual relationship for the serial philanderer Freud in that it did not go beyond kissing and canoodling.
The "Darling Felicity" letters provide glimpses into Freud's life, such as an incident in 1939 when the art school burned down and he was widely thought to be responsible. In one letter, Freud mentions Cedric Morris, the man who set up the school and had been so pleased to have Sigmund Freud's grandson as a student.
The enthusiasm clearly did not last. "Cedric wrote a letter to my mother asking her to persuade me not to come down again as I was too destructive and unscrupulous," Freud writes.
The letters reveal Freud's single-minded focus on his art, and are peppered with references to known works. In one, Freud is animated about all the dead animals in parcels he is getting and making pictures of. "I did one of a gamecock in a bucket of hot scummy water and the fumes and smell of decay was [sic] so overwhelming it sent me into a coma."
That description - he goes on to describe the flies eating the carcass, going mad and diving "with a splash into my paint water where they die" - relates to the 1944 work Chicken in a Bucket. In another letter Freud, who died in 2011 aged 88, talks about his acquisition of a zebra's head: "By far the best thing I have ever bought."
Hellaby, the subject of one of Freud's first full-scale portraits, The Girl on the Quay, has had the letters in her possession for the past 70 years. Now in her nineties, she is selling the letters together with a drawing he made of her in 1941 and two early works he gave to her as presents. They will be sold at Sotheby's on Thursday.