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.

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.