Hunt is an American-born singer who was the inspiration for the Stones' 1971 hit "Brown Sugar" and bore Jagger's first child.
Sotheby's said Saturday that Hunt has tasked the auction house with selling 10 letters written from the set of Jagger's film "Ned Kelly," which was shooting in Australia.
Hunt said the letters chronicling their "delicate love affair" and secret history touch on subjects such as the first moon landing and John Lennon and Yoko Ono.
"When a serious historian finally examines how and why Britain's boy bands affected international culture and politics, this well-preserved collection of Mick Jagger's hand written letters will be a revelation," she said in a statement distributed by the auction house.
Sotheby's books specialist Gabriel Heaton said the letters sent in the summer of 1969 show a "poetic and self-aware" 25-year-old Jagger.
"They provide a rare glimpse of Jagger that is very different from his public persona: passionate but self-contained, lyrical but with a strong sense of irony," Heaton said.
Sotheby's said the collection, which includes song lyrics and a Rolling Stones playlist, is expected to fetch between 70,000 and 100,000 pounds ($111,300 and $159,000) and will go under the hammer on December 12.