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David Letterman is one of the greatest entertainers in the history of television, but his legacy is more fragile than you may think.

That’s because the late night talk show is an ephemeral form, its hosts forgotten faster than teen idols. Onetime superstars like Steve Allen and Jack Paar have faded into obscurity, primarily because it’s difficult to see their shows, and much of Johnny Carson’s oeuvre was erased. Mr. Letterman’s work is not so hard to find, thanks in large part to Don Giller, a superfan who stumbled into becoming a critical custodian of Mr. Letterman’s comedy.

Of the 6,028 late night shows that Mr. Letterman hosted, on NBC and CBS, Mr. Giller, 66, has videos of all but two. His YouTube page has been viewed more than 4.5 million times, and he has become an invaluable resource for journalists on the late night beat. (I couldn’t have written my new biography, “Letterman: The Last Giant of Late Night,” without Mr. Giller’s help.) Even staff members from “The Late Show” have asked him for assistance.

Mr. Giller, who works as a music typesetter for academic journals, has embraced his role as unofficial archivist, but only recently. For decades, he just really liked the show and found that it rewarded his natural obsessiveness.

“You wouldn’t get it unless you watch a couple weeks and notice the story line — to me, there’s a story line,” he said in his Upper West Side apartment. “You start getting his little asides: ‘Oh, he did that a couple weeks ago. That’s what that’s in reference to.’”

To Mr. Giller’s right sat six recording devices stacked on top of one another. To his left was a computer. And all around him were towers of boxes, books and tapes that would make a Collyer brother feel right at home. When I asked if he knows where to find everything in his cluttered apartment, he said, with typical wryness, “I’d like to say, ‘Yes.’”

The origins of Mr. Giller’s Letterman library can be traced, like so many other obsessions, to the assassination of John F. Kennedy.

Mr. Giller, who grew up in Baltimore the son of an accountant, was in seventh grade in 1963. His uncle gave him a reel-to-reel recorder, and the first thing he recalls taping was news of the death of the president.

What began as a diversion turned into a habit. In college, he taped the school marching band, just because, and later he made a point of capturing Academy Awards broadcasts, presidential inaugurations and “Saturday Night Live” episodes. He recently recorded the inauguration of Donald J. Trump, although the internet makes it exceedingly accessible.

But his main focus became “Late Night With David Letterman,” the 12:30 a.m. show that ran on NBC from 1982 to 1993. Mr. Giller recorded it on audio right from its first episode after having enjoyed Mr. Letterman’s short-lived morning show. He later bought a VCR and filled in the holes in his collection through trades with other fans. From the beginning, Mr. Giller took scrupulous notes that formed a Letterman database, with accounts of segments, Top 10 lists, even jokes.

Why record everything? “That’s what I do,” he said. “I always liked making lists and trying to get a handle on something that interests me.”

His compulsion became useful to a larger audience in the 1990s, when Mr. Giller joined early internet message boards dedicated to Mr. Letterman. His exhaustive knowledge marked him as an authority, even among superfans.

“It started when someone asked who played Flunky the Clown,” Mr. Giller said, referring to a cigarette-smoking clown who made cameos on “Late Night.” “I said: Jeff Martin. People were like: ‘Who’s this guy?’”

He became known as the Donz and found friendship in an AOL community made up of what he jokingly described as “like-minded psychotics who are as interested in this guy as I am.” When trolls infiltrated, fans migrated to a private Facebook page, which remains active.

People working at “The Late Show,” which debuted on CBS in 1993, noticed Mr. Giller on message boards and contacted him in 1995. They wanted his help tracking down old Academy Awards broadcasts, to aid Mr. Letterman as he prepared to host the show.

When the talk show started its own website three years later, it went live with a Top 10 list, “Signs You’re Spending Too Much Time Online.” From his desk, Mr. Letterman finished with: “No. 1: Your name is ‘The Donz.’”

Mr. Giller has met Mr. Letterman briefly a few times, the first when he approached him outside the Ed Sullivan Theater in 1994 with a piece of dental floss, a reference to a joke made the night before.

“I made one of the biggest mistakes a citizen can make with a comedian: I cracked a joke,” he said. “I blew it.”

He said he hopes to have a longer conversation, but if not, that’s fine. “I’d like him to know what I did,” he said. “The question is: Will he give a crap? I need to be prepared that he won’t and accept that.”

Mr. Giller said he is not at a loss now that Mr. Letterman is off the air. He is still fielding requests from fans, and hopes to be done digitizing all of “Late Night” by 2018. Pointing at his wall of tapes, he said, “This thing has now become 24/7.”