This is not the first such book. For example, Paul Stump's "The Music's All That Matters" (http://amzn.com/0704380366) covers very similar ground to the description of this book. A quick glance through "Related Books" at amazon suggests such titles as:
Beyond and Before: Progressive Rock since the 1960s by Paul Hegarty
"At last, here is a book that gives prog its due respect as a vital part of the history of rock music, without tying it to a simplistic narrative of over-ambition, decadence and decline."
The Strawberry Bricks Guide to Progressive Rock by Charles Snider
"We'll trip through concept records, indulgent solos and epic album-side long compositions in a definitive reappraisal of rock's most cerebral and renowned genre."
It's nice when friends write a book about a subject we're interested in, but this is 2013 and the test for a phrase like "the first" or "the definitive" is a few keystrokes away ...