Theodore Roosevelt wrote 36 books, as well as numerous articles and countless letters.  He was known to read a book each day!  Many of his books focus on nature and the outdoors, including:

Hunting Trips of a Ranchman (1885)
Ranch Life and the Hunting Trail (1888)
The Winning of the West (1889-96)
The Wilderness Hunter (1893)
The Deer Family (5 Chapters by TR) (1902)
Outdoor Pastimes of an American Hunter (1905)
Good Hunting (1907)
African Game Trails (1910)
Life Histories of African Game Animals (with Edmund Heller)
Through the Brazilian Wilderness (1914)
Stories of the Great West (1914)
A Booklover’s Holiday in the Open (1916)