Historian Robert Caro has spent almost 40 years studying and writing about President Lyndon B. Johnson. The result of that toil, in addition to two Pulitzer prizes, is about 3,388 pages so far on Johnson's life.

The fourth volume, "Passage of Power," has just published.

In the web exclusive video above, Caro talks about why Johnson could get things done in Washington and gives an example of what he calls Johnson's legislative genius in action....