Before Tina Fey, before Wanda Sykes, there was Julia Louis-Dreyfus’s Elaine Benes. So Jennifer Keishin Armstrong believes. In Seinfeldia, her instant bestseller about TV’s most successful show, Armstrong makes the case that Elaine was a pioneering, sex-positive feminist. “Absolutely, she was so liberated in a weird way that we almost didn’t think about it,” Armstrong enthuses, over the phone from her Manhattan apartment.

“Elaine was just one of the boys when she was on that show, without losing the fact that she’s a woman. There are definitely storylines where she is doing women-specific things, like the spongeworthy episode, or the one she’s worrying about whether the women in her nail salon are talking about her behind her back. But she hangs with the guys and is very equal to them in the comedy and everything else.”

Armstrong, who is as entertaining to speak to as she is to read, says that Elaine is the character she most identified with. “Elaine really spoke to me. She’s not thick, she’s beautiful, but not this perfect figure. She’s funny, and she’s actually the most successful of them. She has an actual professional life, real jobs that we know of, though she’s not always perfect at them.”

To Armstrong, Larry David and Jerry Seinfeld are like Lennon and McCartney: “There’s something super special about the two of them together.” She dismisses the long-standing debate that Larry David missed out by not played the role of George Costanza, which went instead to Jason Alexander: “Jason is so great … I don’t think Larry was exactly ready for primetime as a performer. It was enough to be [head] writing the show. It’s a lot of work.

The Junior Mint episode: (upper left) Jerry Seinfeld as Jerry Seinfeld, Michael Richards as Cosmo Kramer. Photograph: NBC/NBC via Getty Images

“And then he could come into his own and do his thing with Curb Your Enthusiasm. Many people have described Curb as Seinfeld on crack. It’s way more intense. I think for the time and the situation, which was primetime network television in the 90s, having that balance of the two of them was very helpful. I’m not sure it would’ve been the mass hit if it came out looking like Curb from day one.”

Armstrong’s previous books include 2013’s Sexy Feminism and Mary and Lou and Rhoda and Ted, about the Mary Tyler Moore Show. (Previously a staff writer for Entertainment Weekly, she writes a television column for the BBC.) She now feels more confident about the challenge of writing books. “It’s harder than you think it’s going to be,” she says. “You just have to get out there and do it. There’s a point with every book where you look at it and you go: this is terrible, this is not a book, I can’t believe I thought I could do this, I’m going to have to give back the advance. You get really dramatic, and then if you stay with it, it somehow starts to work.”

In her book, Armstrong gathers some fresh insights, tracking down writers, directors and bit part actors who have not been interviewed much. She’s proud of her scoop that Saturday Night Live creator and executive producer Lorne Michaels originated the Elaine dance. She is also proud to have talked to Monica Yates, whose father, the novelist Richard Yates (author of Revolutionary Road), inspired the character of Alton Benes, the bad-tempered author with whom Elaine, Jerry and George spend an excruciating evening in the 1991 episode The Jacket. “She had dated Larry David back in the day so she’s the original Elaine,” Armstrong explains.

“I think TV can be an art form, not just stupid throwaway trash,” David once said. Armstrong points out that HBO seemed to learn the most from Seinfeld’s success, giving creators creative license and reaping the results. “A lot of people from Seinfeld have ended up there. Not just Larry, and of course a lot of former Seinfeld writers, on Curb Your Enthusiasm. Veep is now run by David Mandel who is a former Seinfeld writer, and has some fellow writers there. Of course Julia Louis-Dreyfus stars on it, too. Silicon Valley, same deal.”

Seinfeld shunned American television’s obsession with purely likeable characters, paving the way for Walter White and Tony Soprano. “They prepared America for this idea that we don’t need golden heroes. That was a huge concern of the networks at first with Seinfeld, and then it turned out all middle America liked it,” Armstrong says. “Doing it in this comedic way kinda snuck it in on us. Part of why I think some people don’t like the finale is because it was poignant, it said: Ha! You’ve been watching selfish, flawed people for the last nine years and we tricked you into it.”

She passionately defends the much maligned final episode. “I really admire that Larry David made a statement with that finale, and that it involved one of my favourite characters, Jackie Chiles.”

Armstrong says Seinfeld is “shockingly still relevant” after 20 years. “Many, many, many people tell me they have to watch it every night before they go to bed.” Seinfeldia records the show’s extraordinary afterlife, including Seinfeld-themed baseball games. “When you go out to these events, you see how vibrant the enthusiasm is still for the show. It’s more than nostalgia.”

Seinfeld: ‘It’s more than nostalgia.’ Photograph: Seinfeld - The Chinese Restaurant episode/YouTube

Most important, Armstrong says, is Seinfeld’s philosophical legacy: “A great underdog story about creative freedom.”

She finds it inspiring that David and Seinfeld went from just getting by to world domination. Indeed, David first developed Seinfeld while living in Midtown housing heavily subsidised for struggling artists. “Larry David and Kenny Kramer might be living across the hall from each other in Brooklyn these days. They might get priced out of there soon.”

She doesn’t agree with Seinfeld’s critics that the series glorified selfishness. “Seinfeld makes fun of these characters. I don’t think they just get away with things and that it’s saying you can just do whatever you want. One of my favorite things the writers told me was that Larry had told them use stuff from your real life, but have the characters do the thing you wish you had done in the moment. And that makes sense to me as the main point of the show.”