To hear and see Edith Piaf sing press the play > button
According to an excerpt printed in the auction catalogue, Piaf wrote, using a pet name for the actor: "I love you like I have never loved. Taki, don't let my heart die!"
She told Horn in a telegram she sent later that she had never loved anyone as much.
"She said she would give it all, that she needed him ... The whole letter is passion and desperation," said Greek art collector and auctioneer Petros Vergos. "No woman has ever said such things to me."
Vergos said the affair waned either because Horn, six years her junior, did not share her passion or because they both had other partners.
The letter, telegram and a programme of her performance in Athens will be auctioned at the Plaza hotel in the Greek capital. Coincidentally, the sale takes place exactly 11 years after the death of Horn on January 16, 1998.
Vergos said he did not expect the bidding to rise much beyond its reserve price of 1,000 euros ($1,300).
"It is not among the things collectors in Greece chase. It is a cute, a funny thing," Vergos said.
Piaf had the stage name "La Mome" or "little waif" because of her diminutive stature. The film "La Vie en Rose," which netted an Oscar last year for French actress Marion Cotillard in the role of Piaf, revived interest in her life and music.