The fifth volume is expected to cover Johnson's first full year as president, 1964, and continue through the end of his administration in 1969 and his death four years later.

"It is huge," Caro says of the scale of the final book.

Measuring his progress is hard because he doesn't work chronologically. Two years ago, Caro spoke of writing about the year 1967, a time of growing unrest in Black communities and rising opposition to the Vietnam War. Interviewed recently to promote "Turn Every Page," a documentary about Caro and his editor Robert Gottlieb, the author said he is now deep into a section on health care for the elderly before Johnson signed the Medicare and Medicaid Act in 1965.

Caro has always thoughts of his books as not so much the portrait of a man, but of political power and its effects. Taken together, the already published Johnson volumes -- which began with "The Path to Power" and include "Means of Ascent" and the Pulitzer Prize-winning "Master of the Senate" -- exceed 4,000 pages and feature extended probes into everything from filibusters to the mechanics of a stolen election.