I've read the book, and Art and I have discussed it a little bit by private email. It's a frustratingly speculative book. Pierson wants to dismiss all military arguments for the mutiny, in lieu of promoting political and class factors -- but like Art says, and Pierson admits, the evidence simply doesn't exist. To bridge this insuperable gap, Pierson is content to draw broad conclusions from weak induction. He left me unpersuaded, to put it mildly.
There are ancillary points as well. Pierson seems to think that his readers will find it incredible that a sizable unionist population existed in N.O. or other urban centers in the South. He is also very well disposed toward Butler's administration of the city's occupation, but these effective measures (improving health, feeding the poor, etc.) have been documented already, too.