Rolling Stone journalist Sabrina Rubin Erdely spent five months investigating a shocking claim of a gang rape at a University of Virginia fraternity, and the 9,000-word account of the brutal attack published online on Nov. 19, 2014, sent a tremor through the Charlottesville campus and beyond. Then, on Dec. 5, at 1:54 a.m., Erdely sent an e-mail to the magazine’s top-tier editors, Will Dana and Sean Woods, with a simple subject line: “Our worst nightmare.” The body of the message detailed how Erdely no longer trusted the primary source for the most striking anecdote in her article: a U-Va. junior...