Buck in this case is Gen. Buck Lanham. Hemingway followed Lanham and several different infantry divisions through some of the bloodiest battles of World War II.
"Dearest, So what to write tonight. We're in the 6th day of the fight and it has been the mother and father of all fights today with a driving rain trees coming down as in a hurricane," he wrote.
Hemingway later expressed his discomfort at being so close to the fighting, saying: "I'm so damned anxious not to be killed Pickle."
"If there ever was a person who had a finger on the pulse of his times it was Hemingway," Spanier said. "To read his letters is like reading a day-by-day eye-witness account of the 20th century. You are there in the forest, you feel what it is like to be in the middle of a bombardment."
The Finca Vigia collection bears out that observation. Hemingway's letters to Mary are peppered with his observations from the front lines.
"You would have loved the sight of the battle, everything as clear as in an old time battle painting," Hemingway wrote, "Today big fight in woods, ditto yest., day before same…Kraut tough, smart very professionally intelligent and deadly. We will kill and destroy same."
Mary Hemingway once observed that her husband "never discarded anything but old magazine wrappers and three-year-old newspapers." That is evidenced by the diversity of items in the Finca Vigia collection.
There are letters and telegrams from writers and movie stars. Sinclair Lewis wrote to praise Hemingway's writing: "Jesus, that's a great book, 'Bell Tolls.'" Marlene Dietrich sent a "million kisses" from the Plaza Hotel in New York to her "Dearest beloved Papa."
And Ingrid Bergman, who starred in the film version of "For Whom the Bell Tolls," wrote about trying to conceive twins with her husband Roberto Rosellino. A subsequent telegram announces the arrival of twin daughters, one named Isabel.
The collection also offers a window into Hemingway's often complicated relationship with women.
"You are right about women being good friends," he wrote, "They are good friends and dreadful enemies; crazy and completely unreasonable every 28 days and off and on whenever it hits them. They always have to be jealous of something and if you give them no cause for jealousy except your work they will be jealous of that."
And, in a letter to Mary, Hemingway colorfully referred to one of his past relationships: "Pickle it is so wonderful to love someone who is not as unreasonable as a bitch giraffe from Bryn Mawr in heat ..."
"A lot of people bring a very stereotypical view when they come to Hemingway, because he was a larger-than-life character ... but what people don't realize is how really complex he was," Spanier said.
Hemingway left Cuba in 1960 just after the Cuban Revolution. When he died in 1961, many of his personal papers, dozens of his original manuscripts and other personal effects remained on the island at his home, the Finca Vigia.
Mary Hemingway appealed to President John F. Kennedy to travel to Cuba to retrieve the belongings and, with Castro's permission, she was able to ship crates of papers out of Cuba to the United States. Several years later, Mary Hemingway decided to donate Hemingway's papers to the John F. Kennedy Library.
The 3,000 documents and letters from the Finca Vigia collection are available for viewing by appointment.