Frederick Wiseman’s voluminously fascinating documentary “Ex Libris” is ostensibly about the New York Public Library system and its many branches, but, like all of this 87-year-old director’s best work, it’s about a great deal more than its stated subject.

Since 1967, with “Titicut Follies,” Wiseman has looked deeply into the lives of the people who regulate and inhabit metropolitan hospitals, domestic violence crisis centers, dance companies, art galleries, state legislatures, public colleges, meatpacking plants, zoos, high schools, boxing gyms, welfare offices, juvenile courts, and so much more.

He does not tell us how to think about what he puts before us. There are no off-screen or on-screen interviewers in his films, no identifying labels to guide us, no tacked-on music soundtrack. Without any overt editorializing on his part, he and his longtime cinematographer, John Davey, want us to comprehend these institutions as repositories of experience. The slow accretion of detail in his films is essentially novelistic. They offer up a full-scale immersion in the human comedy.

I have been writing about Wiseman in these same terms for so long that at this point, I fear sounding generic. But it’s not my fault that he has been so consistently first-rate over a span of more than 40 films. I don’t know any other living director, of fiction or nonfiction films, who has created a comparable body of work. And I don’t know of any other director who has made so many exceptional films well into his 80s.

The New York Public Library has 88 branches stretching throughout Manhattan, the Bronx, and Staten Island, almost a dozen of which Wiseman features, including the main Beaux Arts structure on 42nd Street and Fifth Avenue with the famous stone lions fronting the entrance. The NYPL is an especially resonant subject for him because it brings to the fore his lifelong theme: the necessity for human connection. For Wiseman, the basic question of what exactly is a library in these digital days becomes a latchkey to a wider world. To him, libraries are finally a celebration of a free society.

It should come as no surprise that the notion of libraries as repositories of books is now quite quaint. In fact, we hardly see any old-school books – or as they are sometimes referred to here, “physical books” – at all. What we do see are the ways in which the NYPL functions as a gateway to a fuller life for an extraordinary range of people. (Among other things, “Ex Libris,” which runs 197 minutes and was filmed in the fall of 2015, is an ode to New York and the shimmering diversity of its people, as was Wiseman’s equally fine “In Jackson Heights” a few years back.)

One library staff member refers to the library as “a warm, welcoming place that’s committed to education and committed to nurturing everyone’s passion and curiosities.” Another, Dr. Khalil Gibran Muhammad, says, “What we do is mind-building, soul-affirming, lifesaving.”

This may sound like empty rhetoric until you see what is actually at work in these branches. There are community forums and job fairs that bring in recruiters from the New York Fire Department, the US Army, and the US Border Patrol. Seniors take part in dance classes. In book clubs, they discuss Gabriel García Márquez’s “Love in the Time of Cholera,” and the weight of the years that they bring to their reading testifies to their unquenchable curiosity. On the other end of the age scale, preschoolers join in singalongs; those a bit older are taught how to read. (As of 2015, more than 1 in 5 New York residents do not have internet at home.)

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In the Harlem branch, a man describes how he could not afford film school and so “I learned from the library.” A sign language interpreter for theatrical productions demonstrates two very different ways of expressing the preamble to the Declaration of Independence. A fielder of phone calls calmly explains to a questioner that “a unicorn is actually an imaginary animal.” The curator of the library’s vast photographic library describes it as a hands-on resource for people who make things and adds, “Andy Warhol stole lots of stuff from us.” We see donor banquets, and we see homeless people sleeping in the stacks. Classical string quartets concertize. Rap artists expound, as do such notables as Elvis Costello, Patti Smith, Richard Dawkins, and Ta-Nehisi Coates. At the Andrew Heiskell Braille and Talking Book Library on West 20th Street, we see a blind library volunteer show a newly blind patron how to read Braille.

Ultimately “Ex Libris” demonstrates that libraries are about people, and what gives the film its great and accumulating force is that people are infinitely complex. At least, in Frederick Wiseman’s films they are. Grade: A (This movie is not rated.)