Mr. Sandusky and his wife adopted six children, and knew many more through their charity, Second Mile. Mr. Sandusky always had kids buzzing around him, even when there was Penn State work to be done. Mr. Posnanski says about Paterno’s reaction: “The kids annoyed the hell out of him.”

Mr. Sandusky turned out to be consummate evil. This past June he was convicted of 45 counts of the sexual abuse of young boys. How could this have gone on for so long without Paterno’s knowledge? At least one instance of molesting was clearly reported to Paterno in 2001 — details are fuzzy about how much detail Paterno absorbed — and all the coach did was report the case to the university’s athletic director.

An investigation by Louis Freeh, the former F.B.I. director, concluded that Penn State officials, Paterno included, “exhibited a striking lack of empathy for Sandusky’s victims.” One Penn State employee says to the author about Paterno, in a quotation that rings across this biography: “Why didn’t he follow up? Why?”

Mr. Posnanski does not let Paterno off the hook. He calls Penn State’s and Paterno’s response to early indications of Sandusky’s depravity “sickeningly inadequate.” But he sets Paterno’s inaction in context: he was old, a bit befuddled and — a sin in football — he simply took his eyes off the ball. He’d stayed far too long as head coach, and was not the man he once had been.

The book’s primal moment arrives in its final section. Mr. Posnanski sits alone with Paterno, who has already been fired and has learned he has lung cancer, at his kitchen table. “So,” Paterno asks him, “what do you think of all this?”

Mr. Posnanski writes: “I told him that I thought he should have done more when he was told about Jerry Sandusky showering with a boy. I had heard what he had said about not understanding the severity, not knowing much about child molestation, not having Sandusky as an employee. But, I said: ‘You are Joe Paterno. Right or wrong, people expect more from you.’ ”