The FBI knew that Robert Hanssen, an FBI agent near retirement, was working for the Russians. However, to prove it in court they needed to use evidence not gathered from covert sources they wanted to protect. They assigned Eric O’Neill, who had been working with the FBI group tasked with following people, to Hanssen and put them both in a unit whose real and only reason for being was to give O’Neill the chance to spy on his boss.
“Gray Day” is a tense book that, to use a cliche, “reads like a novel”. Given O’Neill’s low, albeit critical, role, it isn’t the full story of Hanssen’s betrayal, but it is a very good read.