Issroff visited more than 100 shtetls in Lithuania in his efforts to memorialize a world that has all but disappeared.

"Seeing and appreciating the sheer devastation of the country's Jews" transformed "intellectual curiosity" into action, he said.

The result was a four-volume opus, "The Holocaust in Lithuania, 1941-1945: A Book of Remembrance."

"For many of the families there is no graveyard, no tombstone, and a book becomes a way of recording the deaths of these people," he said.

Issroff, a native of Port Elizabeth, South Africa, who lives in London, and Cohen, a Capetonian who resides in Jerusalem, began their effort by studying archives around the world.

"We found that there was material that had never been pulled out and put together, and that's where the concept of creating a database and eventually publishing a record of it came out," Issroff said.

In the foreword, Stephen Smith, the non-Jewish founder of the Beth Shalom Holocaust Center in Nottingham, England, describes the book as a "mitzvah of the highest order. It is about the struggle against forgetting, in the shadow of mass death."