(CNN) -- A rare letter evincing a display of affection between President Theodore Roosevelt and his youngest son is up for sale by a dealer who obtained it from a Roosevelt family friend.
Roosevelt sent the letter to his 6-year-old son, Quentin, during a trip to Yellowstone National Park in 1903. It is the only letter from the trip to his family to reach the market, and its existence was unknown to scholars and institutions until its discovery in the possession of a family friend, said Nathan Raab, vice president of the Raab Collection, which is selling the letter through its Web site.
"The relationship between Theodore Roosevelt and Quentin, his favorite son, is not one many people know about, so finding a letter like this to Quentin is a once in a lifetime discovery made even more poignant by the fact that it's unpublished," said Raab, who values the letter at $25,000.
In the letter, Roosevelt addresses his son by his nickname, "Quenty-Quee," and provides a brief glimpse into life on the trail, including a small sketch of the mule that carried his gear on the trip.
"I love you very much. Here is a picture of the mule that carries, among other things, my bag of clothes. There are about twenty mules in the pack train. They all follow one another in single file up and down mountain paths and across streams."
The letter is signed, "Your loving father."
Raab said, "This offers another side of Theodore Roosevelt, who was this rough rider, men's man, yet had this warm, loving affectionate, relationship with a son, who shared a lot of his father's physical and intellectual attributes. He was the apple of Theodore Roosevelt's eye."
Quentin Roosevelt, the youngest of Edith and Theodore Roosevelt's six children, was 3 years old when his father was elected president. He was known for his rambunctious behavior in the White House and eventually for his scholastic aptitude, drawing comparisons to his father.
He joined the United States Army Air Service and became a fighter pilot during World War I at the nudging of his father, an ardent promoter of the war. He was killed in aerial combat over France when he was 20.
His death profoundly affected the president, Raab said.
"His friends said he was never the same man again, and you see the love he had for his son in this letter," Raab said.