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BBC America's hit series Killing Eve—now streaming on Hulu—is the gift that keeps on giving.

Ever since the show premiered in 2018, it's caught everyone's attention thanks to its gripping, sharp-tongued, and endlessly unpredictable story. Its main plotline? The cat-and-mouse game between a bored intelligence agent (Sandra Oh) in London and the charismatic assassin she's investigating (Jodie Comer). But even fans of the show may not realize that Oh's Eve and Comer's Villanelle aren't new creations—they're actually characters from a pair of novels written by British author Luke Jennings. Here's what you should know about the books Killing Eve took inspiration from.

Killing Eve is actually based not on a novel, but on a series of novellas by British author Luke Jennings entitled Codename Villanelle. The four Codename Villanelle e-novellas were published online between 2014 and 2016, and followed the story of a Russian orphan, Villanelle, who murders her father's killers and is then trained as an assassin by a shadowy organization. Years later, Villanelle is pitted against Eve Polastri, a desk-bound agent at MI5–the British version of the FBI–in a battle of wits that takes the lives of both women in some very unexpected directions. The four e-novellas were compiled into a single novel and released in April of 2018.

Phoebe Waller-Bridge, who broke out in 2016 with her incisive, hilarious Black comedy series Fleabag, was tapped to adapt Jennings's novellas by executive producer Sally Woodward Gentle.

"Sally actually put the novellas in front of me, which is how the conversation started," she recalled at a Television Critics Association Press Tour in January 2018. "I read the first novella and just fell in love with these characters, thought they were just wild. I hadn’t seen them before and just was desperate to stand them up."

Waller-Bridge added that Jennings took a chance on her, since Fleabag had not yet debuted at the time and it was the only project she had written. "But I think [she] thought I was mildly perverse enough," Waller-Bridge joked.

The show's producers and stars have described Jennings's novellas as an inspiration and start-off point, rather than a text they follow closely. "I think the show has definitely a different tone to the novellas," Oh said at the Television Critics Association panel. "I would say in the novellas, the character Eve was much more serious."

Another major difference? The show is not focused on two white women. "Eve was, as I interpreted the novella, was white,'" Oh said. "I am not white. I am Asian. And I will say that I was extremely pleased that that was taken into consideration in the casting–and also wasn’t taken into consideration in the casting, if you know what I’m saying." Eve is also British in the novellas, while Oh's Eve is written as half-British; She was born in Connecticut but has lived and worked in London for many years.

Though the show takes some departures from the book, Jennings was heavily involved in the adaptation process and is thrilled with the series and the casting of Oh, in particular.

"What we had in Sandra was someone who absolutely embodied the spirit of that role and did so with a wonderful humor that nobody else that was seen for the role really saw. She saw the humor in it, the wit, and the self deprecation," Jennings told Town & Country recently. "Sandra is a wonderful actress because she has no vanity about the way that she presents herself. What you see is a very transparent performance. And that’s just perfect for that role."

There is! Codename Villanelle: No Tomorrow was released on March 26, 2019. While the release date tied in with season two of Killing Eve, which debuted on April 7, there's not much overlap between the sequel's storyline and season two.

No Tomorrow picks up with Eve pursuing Villanelle across the globe, having discovered that a senior MI5 agent is on the payroll of Villanelle's bosses, The Twelve. The two women's mutual obsession grows as Villanelle begins to suspect that The Twelve may be about to turn on her, and that working with Eve to defeat them may be her only option. Jennings told EW that he enjoys the times when "the show’s storylines entwine with mine.”

Yes! BBC America confirmed in April that the show will be coming back for a third run. And each season will feature a new female showrunner. Season two saw Emerald Fennell take over from creator Phoebe Waller-Bridge, while Fear the Walking Dead writer Suzanne Heathcote will take over from Fennell for season three.

In addition, it sounds like a third book may be on the way. Jennings told Entertainment Weekly that he sees No Tomorrow as part two of a trilogy.

“As Eve locks onto Villanelle’s trail, I wanted to show that pursuit, no matter how pitiless, is also a kind of courtship,” he said of No Tomorrow. “In the end, it’s just the two of them, playing by rules only they understand.” And in this aspect, the books and the series are absolutely in sync.

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Emma Dibdin is a freelance writer based in Los Angeles who writes about culture, mental health, and true crime. She loves owls, hates cilantro, and can find the queer subtext in literally anything.