Published editions of letters and correspondence are a useful resource for the writings of scientists and their contemporaries.
Select recent editions are shown here, but keep in mind that the History of Science Collections has many first and/or early printed editions of correspondence.
Beyond Innocence by 
Correspondence of John Wallis, 1616-1703 by 
George Washington Carver In His Own Words by Listed here are select modern editions of diaries or journals, another type of primary source useful for research.
The Diario of Christopher Columbus's First Voyage to America, 1492-1493 by 
The Private Journal of William Reynolds: United States Exploring Expedition, 1838-1842 by 
