He started working there as the host of an afternoon show on MSNBC, “Ronan Farrow Daily,” in 2014. After it was canceled in 2015, he started reporting investigative segments for “Today.” In 2017, he embarked on his investigation of Mr. Weinstein.

For years, stories had circulated about the alleged transgressions by Mr. Weinstein, who has denied ever having had nonconsensual sex, but no journalist had been able to work them into publishable form. At several points early in the reporting process, Mr. Farrow writes, his bosses seemed less than enthusiastic about his leads.

He writes that Mr. Oppenheim had a habit of “scrunching his nose and holding journalism at arm’s length” when discussing the Weinstein investigation. He says the head of NBC’s investigative unit, Rich Greenberg, told him to put the story “on the back burner” at one point. Mr. Greenberg disputed that assessment.

Mr. Farrow also describes “tiptoeing” around NBC producers and executives with Mr. McHugh, the producer, as they gathered more material. Mr. Farrow likens the situation to a Catch-22: He needed more reporting, but he no longer felt comfortable doing it openly.

Mr. Farrow came into possession of a portion of a tape recording, made during a New York Police Department sting operation, in which Mr. Weinstein admitted to the Italian model Ambra Battilana Gutierrez that he had groped her. With that, Mr. Greenberg became more enthusiastic, saying, “If this airs, he’s toast,” according to the book. Susan Weiner, NBC News’s top lawyer, also seemed on board after listening to the recording, telling Mr. Farrow he could go to Mr. Weinstein for comment, he writes.