Robert Dallek sat in the National Archives day after day, mining the 20,000 pages of Henry Kissinger’s telephone transcripts for historical gold. And every so often, amid the blur of bureaucratic tedium, a little nugget would glitter. One was the Nixon-Kissinger phone call reacting to news of the 1973 coup in Chile that overthrew Salvador Allende, whose Socialist government they had worked covertly to undermine through the C.I.A.

Mr. Kissinger grumbled to the president that American newspapers, “instead of celebrating,” were “bleeding because a pro-Communist government has been overthrown.”

“Isn’t that something?” Nixon remarked.

“In the Eisenhower period, we would be heroes,” Mr. Kissinger said.

“Well, we didn’t — as you know — our hand doesn’t show on this one,” the president said.

This brilliant, devious duo is glimpsed in a moment of gloating camaraderie, even as Watergate was bringing the presidency down around them. History, Mr. Dallek said, resides in such details.

Nixon is the fifth president to come under the scrutiny of Mr. Dallek, author of generally acclaimed books on Franklin D. Roosevelt, Lyndon B. Johnson, John F. Kennedy and Ronald Reagan. This time he has chosen to pair the president with the adviser he describes as a “kind of co-president,” surmising that each would be a foil for the other. The 700-page result, “Nixon and Kissinger: Partners in Power,” to be published next week by HarperCollins, shows that their extraordinary relationship was as much rivalry as partnership, as two driven men sparred over which of them would get the limelight both craved....