There's a billion good ideas in the world, but success comes to those who can execute a good idea consistently. The case studies featured in "Testify!" are a few that have carried out their customer evangelism ideas. The organizations profiled believe in customer evangelism as a theology and have the results to show for it in practice.
When they say compiled, they literally mean compiled. They didn't add any commentary or write-up for the companies, just compiled letters and reports that were sent in. If you've read the original book, this one doesn't add that much except to provide individual company testimonials.