In "Undaunted Courage," Stephen E. Ambrose uses New Historicism. What is New Historicism?
New Historicism refers to a form of literary criticism that emerged during the 1980s and made popular in the 1990s by authors such as Stephen Greenblatt. Whereas older forms of literary criticism focused its attention on how a literary work reflected the time in which it was produced, New Historicism also considers the perspective of the work's author. It assumes the author's social, political, and economic background all contribute to his/her perspective. In addition, the psychological perspective of the author and other influences on the author's outlook are also important considerations in New Historicism. While older forms of literary criticism may have considered these issues, they serve as the focus of New Historicism.
The result of this outlook is a more relativistic view of a literary work's relationship to the period of its production. A work can represent aspects of a period - those aspects reflective of the author's perspective. In this idea is the assumption that an author of the same period can produce an entirely different work that reflects his/her own influences. Taking Shakespeare as an example, a New Historical interpretation would not focus solely on how Richard III reflected Tudor animosity toward the House of York; it would also take into consideration what Shakespeare read (Sir Thomas More's History of Richard III), as well as his political relationships as influencing factors.