The death last month of one of the nation’s top experts on Abraham Lincoln has devastated the “Lincolniana” world, taking from it a “walking encyclopedia” of everything about the former president.

According to friends and New York City police, Jonathan Mann, 61, was taking a regular midday walk across the Manhattan Bridge on Aug. 5 when a man allegedly approached him, said it was his day to die, and beat him. Police have arrested a 41-year-old man from Queens.

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Mann was taken to Bellevue Hospital and released to convalesce at a friend’s home in Shelter Island Heights. Days later, as they were walking and talking about getting ice cream, Mann collapsed into the arms of his friend, James Olinkiewicz.

“This is a man who walks 10 miles a day,” Olinkiewicz said. After a short walk down the driveway, Mann asked to turn back to the house. “He took three steps, grabbed my shoulder and said, ‘I can’t breathe,’ and dropped dead in my arms,” he said. “I’ve been having nightmares,” added his longtime friend.

Photo courtesy Provenance Productions

So has the Lincoln world, where Mann was a premier collector and trader, celebrated for orchestrating a huge exhibition at Federal Hall titled “Lincoln in New York” in 2009, the year the nation celebrated the former president’s bicentennial.

“He was very important in our world,” said Bill Panagopulos, president of Alexander Historical Auctions. “He was a walking encyclopedia. He knew everything about Lincoln and everything associated with Lincoln, probably among the top Lincoln scholars in the country,” Panagopulos added.

“This is a great loss,” said Peter Klarnet of Christie’s New York. “I’m not sure how that gap is going to be filled,” added the Americana specialist.

Mann made a name for himself in the collector’s world when he and Donald Ackerman started the Rail Splitter, a publication about Lincoln collectibles, over 20 years ago. “He considered it part of his legacy,” said Ackerman.

Known to have a wide group of friends, Mann, Ackerman, and Olinkiewicz anchored the 2009 Federal Hall exhibit, “Lincoln In New York.” Mann was able to tap collectors for rare items including a signed copy of the Emancipation Proclamation and even talked the Library of Congress into lending items for the show that attracted nearly 200,000 people.

To promote the exhibit, Mann won an appearance on The Martha Stewart Show where she was wowed in seeing and touching some of the items he brought in, including the veil former first lady Mary Todd Lincoln wore the night her husband was assassinated at Washington’s Ford’s Theatre.

To Stewart, Mann called Lincoln “arguably our greatest president” and added that he was “the quintessential American story.”

Recently, Mann added documentary-making to his passions, helping form Provenance Productions in New York. The Public Broadcasting Service featured one of their films, The Oratorio. Said Ackerman, “He managed to persuade Martin Scorsese to appear in the film and give permission to incorporate clips from some of his films, no small task!”

Provenance partner Mary Anne Rothberg told us that in the weeks before his death, Mann was working on a film about the legacies famous people including author Tom Wolfe and astronaut Jim Lovell hoped to leave behind.

“It wasn’t doing well in terms of sales and so forth. And we decided to reedit it and make Jon the central character in the movie,” she said. “So he’s exploring the meaning of life and sort of struggling to come to terms with what it’s all about. And we were working on our final edits when Jon unexpectedly died, definitely ironic given what happened,” Rothberg said.

Doubly ironic was where he was beaten. Olinkiewicz said Mann liked walking across the Manhattan Bridge because of its views.

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On his Provenance profile, the last sentence reads, “When not geeking out, he can be found (weather permitting) walking across the Manhattan Bridge to Brooklyn.”

Rothberg said, “That was definitely one of his favorite things to do. And, tragically, we know how that turned out.”