From The Feminine Mystique to Rosemary’s Baby, from Portnoy’s Complaint to The Penny Wars, the creators of Mad Men have squeezed in references to some of the most celebrated literature of the 20th century. On Sunday night’s premiere, we get our first reference when Don and Roger are served by a diner waitress with a copy of John Dos Passos’s U.S.A. trilogy, published in the 1930s, tucked into her apron pocket. (“Do you have anything by John Dos Passos?” Roger teases her.) Billy Parrott, managing librarian of the art and picture collections at the Mid-Manhattan Library, has been chronicling the meanings behind some of Mad Men’s most iconic literary references on his blog for the New York Public Library, The Mad Men Reading List, over the past five years. On the U.S.A. reference, Parrott noted, “It’s that time period where things change. It was the end of innocence [for] that particular generation.” Like the song that bookends this episode, the trilogy is a perfect fit for Mad Men’s themes, and as Matt Zoller Seitz points out, an acknowledged influence on the show.

Parrott took Vulture through some of the best literary references on the show so far, and what he predicts we might see this season.

“I like the ones where instead of just showing them reading, the actual books make their way into the show. They did that with Frank O’Hara’s Meditations in an Emergency [in season two]. Some of those poems were the dialogue, voice-overs, or closing credits of some of the scenes.”

“They also did it for the beginning of season six, where Don is reading Dante’s Inferno. There’s a voice-over, the first opening sentences of the book, which really relate to the show. When they incorporate the text into the show, it’s so spot-on.”

Midway upon the journey of our life

I found myself within a forest dark,

For the straightforward pathway had been lost.

“The other one is a little quirkier. There are two different books. Betty Draper is reading “The Diamond As Big As the Ritz,” a short story by F. Scott Fitzgerald.”

“A little later in the season, Sally is reading The Twenty-One Balloons, the children’s book that came out in the ‘40s. Seeing the show, unless you really know what that book looks like, you only get a quick glimpse of it. You get more of a glimpse of the inside of the book. I went for months not knowing what that book was, and eventually reached out to a NYPL colleague, a children’s librarian, who has an amazing blog. She put it out and said, ‘Hey, children’s librarians everywhere, do you recognize this?’ And within two minutes, people were like, ‘Oh yeah, that’s The Twenty-One Balloons.’ Even better, though, is that when The Twenty-One Balloons came out, William Pène du Bois wrote the book and gave it to his editor and said, ‘Have you read “The Diamond As Big As the Ritz”? It’s the exact same story. You’ll be accused of plagiarism.’ If you read it, there’s an introduction where he says, ‘Yes, there are coincidences, but they’re just a coincidence.’ I think it’s funny that the mom is reading one book and the daughter is reading another, and they’re not the same book, but very close.”

Lady Chatterley’s Lover is passed around the office in season one.

“[Then there are books] that appear because they were popular at the time. Books like Lady Chatterley’s Lover [season one] always have those kinds of followings. If Mad Men came out today, the same characters would be reading and giggling and pointing at words in Fifty Shades of Grey. Every generation, there are those kinds of books. That particular one, they wouldn’t be standing around giggling over Rebecca of Sunnybrook Farm. The subject matter, that’s something that just had everybody talking.”

What might be popular this season of Mad Men?

“Maybe somebody will be reading one of the best-sellers from around that time, The Godfather by Mario Puzo. Would that add anything to any particular character or character development? Probably not. But Pete might read that book.”

Matthew Weiner will appear at “LIVE From the NYPL” on May 20 to discuss Mad Men’s series finale.