I am listening to Wilentz's account of the Iran-Contra affair.
Wilentz's big picture perspective is that Reagan did know the substance of what was happening (in particular, the decision to trade arms to the Iranians -- via the Israelis -- in exchange for the release of seven American hostages being held in Lebanon). He further believes that the Democrats would likely have pursued impeachment if the scandal had not arisen late enough during Reagan's second term that they did not have much to gain -- politically -- even if they succeeded in removing him from office.
One of the major players in the scandal was William J. Casey, who was the director of the CIA from 1981 until 1987. Casey had been Reagan's campaign manager for the 1980 election, and Wilentz argues that Casey, Robert Mcfarlane (head of the NSC), John Poindexter (Mcfarlane's successor), and Oliver North orchestrated the sales to Iran, while North was (individually?) the major instigator in channeling the funds to the Contras.
Casey suffered a stroke just before he was scheduled to testify before Congress about Iran-Contra, and Wilentz implies that had he survived we might know more about Reagan's role in various decisions.