When reading a book on true history, I find that the notes sections are often what makes the overall quality of the work shine through. If I feel compelled to read through the notes, this tells me a few important things. It usually means that the work is well researched, and allows the notes to present a clear basis for the author's statements, as well as alternative interpretations of the evidence. Well written notes give enough context that I don't feel the need to annoyingly flip back and forth between sections. Most importantly, if I'm spending time reading through the notes, it means I didn't want the story to end.
The best non-fiction works have a strong narrative without sacrificing academic rigour. My favourite work of this kind, Batavia's Graveyard, by Mike Dash, is an excellent example of a book I did not want to put down, even while reading the notes. Ghost on the Throne, by James Romm, also passes this test and is easily the best non-fiction book I've read this year.