How do you take a story about an actual serial killer who constructed an actual murder castle--complete with secret passages and rooms and chutes--and make it BORING?
The Chicago World's Fair stuff was more interesting by far than the H.H. Holmes chapters. But the lists and menus he included made it feel like he was stuffing a term paper to get it up to the minimum required page count.
Larson apparently tried to take a historical narrative and spice it up by talking about how people felt or what they thought, and to me it often didn't ring true. I think I'd have enjoyed this immensely if the author had picked a style--biography/historical account or novelization--and stuck with it.