Forty-seven percent of twelfth-graders reported using letters, diaries, or essays written by historical people at least once a month in their studies of U.S. History in 2010.

http://nationsreportcard.gov/ushistory_2010/context_3.asp?tab_id=tab3&subtab_id=Tab_1#chart

As part of the 2010 U.S. history assessment, twelfth-graders were asked about the extent to which they used various resources when studying history.

The percentage of students who responded in each category are shown.

Approximately 71 percent of twelfth-graders reported reading from a textbook at least weekly, while 17 percent reported using with the same frequency letters, diaries, or essays written by historical people.