From an investigative journalist and regular contributor to The New York Times, an eye-opening and wildly entertaining book about the multimillion-dollar fitness phenomenon CrossFit—and about its mercurial founder, Greg Glassman.

How did an upstart workout regimen, begun out of one small gym in Santa Cruz in the ‘90s, become one of the most popular and polarizing fitness platforms in the world? And who, exactly, is the person responsible for its success?

The Glass Box is a page-turning, riveting, and upsetting look at CrossFit—one of the most popular fitness programs in the world—and at its polarizing and mercurial founder, Greg Glassman. Written by veteran journalist Calum Marsh, whose articles on CrossFit in the New York Times have gone viral, it is both a thorough history of a company’s coming-of-age and an eagle-eyed examination of what happens when a health-related endeavor in these increasingly lawless times is led by someone who allegedly forgoes quality control in favor of expansion and market domination.

CrossFit is a phenomenon, and its studios and methods are now ubiquitous. But its extreme workouts can lead to serious health problems, including permanent muscle damage and, in some cases, death. What do we do when a cult of personality shifts into something much more dangerous? And how do we extricate well-meaning followers of this platform from its perilous and sometimes deadly practices? Calum Marsh explains all of this and more in this necessary and engrossing read.