What is the author's style in the biography, The Wright Brothers?

Although The Wright Brothers by David McCullough is a biographical work of Orville and Wilbur Wright inventors of the first successful flying machine, also known as the airplane, the focus of the book is on their work. There is only cursory mention of their personal lives. But that structure is fitting since their personal lives were normal and unremarkable; however, their successes and achievements in aeronautics was nothing short of miraculous. The book is structured in part as an epistolary; many letters exchanged between principals are included verbatim in the narrative.