"The Phantom Tollbooth" isn't the kind of book children buy for themselves; it's the kind they are given. I myself didn't discover it until I was an adult, when a woman I was seeing presented me with a copy with the understanding that I must study it, if I harbored any hope of ever understanding her. It's an odd kid's book anyway, with an overabundance of words and wordplay (by Norton Juster) and sparse illustrations (by the Pulitzer Prize-winning artist Jules Feiffer, best known for his acerbic and satirical cartoons).

None of this has stopped the 1961 chapter book from becoming an out-of-left-field classic, as the documentary “The Phantom Tollbooth: Beyond Expectations” makes clear. Featuring interviews with New Yorker writer Adam Gopnik and other fans — along with Juster and Feiffer — the movie takes an affectionate, if somewhat bemused look at the book’s enduring popularity.

It’s a charming little film, even if you’re not as passionate as the guy in the movie who had himself tattooed with an image of the book’s hero, Milo, and Tock the Watchdog — who, yes, has a timepiece embedded in his side — over his heart.

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“The Phantom Tollbooth: Beyond Expectation” screens Sunday at 3 p.m. in Baird Auditorium of the National Museum of Natural History. Juster will participate in a post-screening discussion of the film with singer-songwriter Bill Harley, whose “Milo’s Tune” is featured in the documentary, and producer Janice Kaplan.

Tickets are $25; $20 for members of the Smithsonian Associate; $10 for children younger than 15. 202-633-3030, www.smithsonianassociates.org.

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