I feel like I recommend this book all the time, but it is almost sui generis, and very excellent: Peter Novick, That Noble Dream: The “Objectivity Question” and the American Historical Profession (Cambridge University Press, 1988). Not only does Novick tell the story of how "Historian" became a profession, rather than an elite recreational activity, but also tracks the idea of 'objectivity' as history evolves within the United States. I must warn you though, it is very thick! If you already have a vague familiarity with the names involved, it can be quite juicy, almost gossipy at parts as you read about the spats and scandals between historians over the course of the twentieth century.