What defines quality of an open-education book?

I’m dubious about open-education resources, because I see that the capitalist system does encourage authors to produce books that teachers want and that work for students (books don’t remain adopted if students hate them).  I’m willing to believe the claim below that “free, high-quality educational materials are available” but I’d like to know the definition of “high-quality.”  Who measures the quality of open-education resources?  By what definition can we claim that OpenStax books are high-quality?

I’m working on an answer to this question for the ebooks we’re working on.  While I am an author of textbooks with commercial publishers, I am also writing books for the Runestone Interactive site.  For those books, we are developing assessments to measure learning pre/post, and we are measuring retention and student engagement.  I hope to have evidence to support the claim that our books are high-quality.

What kinds of processes are open-education resource providers using to make a claim about high-quality?

“Although a growing number of free, high-quality educational materials are available, many instructors and academic leaders are uncertain about how to begin taking advantage of these resources,” said David Harris, editor-in-chief of OpenStax College.

With a combination of high-quality content and a well-supported pathway to OER adoption, OpenStax College and Lumen Learning expect to achieve significant textbook cost savings at both two- and four-year colleges and universities nationwide.

via Open-education partners hope to save students $10 million by 2015.