I will believe it when I see it.

My Goodreads review of the book:

This best-seller has a surefire double subject (a World's Fair and a serial killer!) and is quite absorbing; but I think that by historians' standards, Larson cheats (which is to say, makes stuff up, including dialogue and descriptions of people's observations and thought processes) in the interest of providing a novelistic narrative. So although I enjoyed the book reasonably well, I do not respect the author's journalistic integrity. He takes the easy way out time and again.

There is a kind of social capital to be had from reading this sort of non-fiction bestseller because others will have read them or at least heard of them. But there is an enormous variability in their quality, and this is at best one of the middling ones.