(CNN) -- For five generations, the Meserve/Kunhardt family has been the collector and custodian of some of the most valuable photographs and memorabilia of Abraham Lincoln.

Peter W. Kunhardt, left, Philip B. Kunhardt III and Peter W. Kunhardt Jr. co-authored "Looking for Lincoln."

In fact, eminent Lincoln historian Harold Holzer said there's only one other family that's contributed more "to our understanding of America's most enduring leader," and that's the Lincolns themselves.

"The Kunhardts -- and before them, their ancestor Frederick Hill Meserve -- have been active, and crucial for generations, in preserving Lincoln's image for posterity," Holzer said.

"Mr. Meserve literally saved photos from destruction, then cataloged and analyzed them. Later members of the family wrote seminal books incorporating the family-owned archive into the stories of Lincoln's life and death."

The family collection was the basis for the new book, "Looking for Lincoln" (Knopf), and for a PBS documentary. One of the book's co-authors, 26-year-old Peter W. Kunhardt Jr., has now joined the family business as assistant director of the Meserve-Kunhardt Foundation.

Kunhardt, who recently earned a master's degree in art business and photography, also works on the Gordon Parks Foundation, a division of the Meserve-Kunhardt Foundation, which includes the life's work of the pioneering African-American photographer.

Kunhardt spoke with CNN in an interview and follow-up conversations about the extraordinary family history.

CNN: Tell us about your family.

Peter W. Kunhardt Jr: I'm the fifth generation to be working on Lincoln. My great-great-grandfather was a man by the name of Frederick Hill Meserve. His father, William Neal Meserve, was a soldier in the Civil War who was wounded at the battle of Antietam, and he kept a diary of the entire time he was in war.

That diary was illustrated by his son, Frederick Hill Meserve, years after the war through collecting photography. He became one of the leading collectors of 19th-century American photography. ... At that time photography was still quite new.

In the late 19th century, the war-weary nation lost its interest in the images of that era. Many glass negatives were thought to be worthless. The emulsion was washed off, and many were used to build greenhouses.

Meserve wrote a book in 1911 called "Photographs of Abraham Lincoln." He became obsessed with Lincoln. Lincoln was the first president to be photographed while in office. Meserve's goal in life was to find and catalog all the existing images of Lincoln, of which we now know there were about 120.

Each photograph in Meserve's book, which he updated for the next half century, was given an "M" number. Part of our work was to update his numbering system and establish a new national standard.

Then his daughter Dorothy Meserve Kunhardt, she became heavily involved with Lincoln scholarship and the collection as well. She was the author of the children's book, "Pat the Bunny," which ... as everyone knows today, it's one of the most popular children's books. She worked with her father on Lincoln publications and continued to collect Lincoln.

The collection was passed to Philip Kunhardt Jr., my grandfather, who eventually passed this collection of photographs on to my father and uncle, who were my co-authors in this book, "Looking for Lincoln."

My grandfather, father and uncle wrote a book on Lincoln in 1992 called "Lincoln: An Illustrated Biography," using the contents of the collection.

Each generation has continued the fascination with the collection but with a special interest in Abraham Lincoln.

CNN: What's your involvement now in the collection?

Kunhardt: My role is the assistant director of the Meserve-Kunhardt Foundation. The foundation was established in 2002 by my father and uncle in order to preserve our family's collection and make it available to the public.

During the past century, the collection had grown so large that
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