Primary documents are always the foundation of my research. I am known for turning up obscure published and unpublished source materials, and that's what I focused on for this book.
If that quote doesn't grab you, if it strikes a "yeah, yeah, okay" note, you haven't lately read any of the vast sea of Pulitzer Prize-winning history rooted in secondary sources. To paraphrase Tom Rowland (again), the deeper one goes into the material, the greater becomes the shock and personal fear at discovering how dependent one has been on other people's previous surmises.
The risk of not going down Eric's path, of trusting the previous treatments, is that you are made the fool by aggregating bad stuff. And that, in a nutshell, is the central problem in Civil War history today.