A very famous six-volume biography is

Carl Sandburg's blography of Abraham Lincoln

Douglas S. Freeman's blography of General US Grant

Ernest Hemingway's blography of Aaron Burr

Carl Sandburg's biography of Abraham Lincoln

Explanation:

"Abraham Lincoln: The Prairie Years and The War Years" is a quite famous biography of US President Abraham Lincoln, written by Carl Sandburg.

This biography was originally published as a set of six books, first two volumes dealing with Lincoln's childhood and life until presidential inauguration were published in 1926, and last four volumes, dealing with the Civil War period, were published in 1939.

His work us widely considered as one of the best biographical books ever and he recieved Pulitzer Prize in 1940 for it.