A Gallup poll in 2010 assessing the nine presidents from John F. Kennedy to George W. Bush put Richard M. Nixon last with a 29 percent approval rating. In a Quinnipiac University survey at the beginning of July, however, Barack Obama had replaced Nixon as the worst president since World War II: 33 percent assigned that unwanted distinction to President Obama; only 13 percent thought Nixon was that bad.

Present troubles go far to explain Mr. Obama’s low standing, but historical amnesia, especially among people over 50 who have direct memories of Nixon’s resignation 40 years ago next month, seems particularly surprising. Nothing has contributed more to current American cynicism about politics and politicians than Watergate or Nixon’s betrayal of the public trust that cost him the presidency. Tapes revealing his appalling vulgarity in response to anyone he considered an enemy have permanently tarnished the presidency.

It is well, then, that John W. Dean has written “The Nixon Defense: What He Knew and When He Knew It,” his third book on the scandal, or more precisely, the cover-up that brought Nixon down. Mr. Dean’s book will remind people of why Nixon deserves so unflattering a historical reputation, despite the opening to China and détente with the Soviet Union. It should also serve as a renewed cautionary tale about elevating politicians with questionable character to high office.

Mr. Dean, the counsel to the president from 1970 to 1973, went to prison for his part in the cover-up, but won a reduced sentence by cooperating with prosecutors and made further amends in two previous books, “Blind Ambition” and “Lost Honor.” Both are valuable additions to the Watergate literature, though the 2005 revelation that “Deep Throat” was the F.B.I.’s W. Mark Felt refutes Mr. Dean’s assertion that Alexander M. Haig Jr. was the leaker.