I've only read the first two novels, 'Codename Villanelle' and 'No Tomorrow', but from what I've seen, the expansion is more about psychological texture than plot sprawl. The books, especially the first, spend a lot of time inside Villanelle's head in a way the show only hinted at in early seasons. You get her cold, almost alien internal monologue about her kills and her fascination with Eve, which feels less playful and more genuinely pathological than Jodie Comer's portrayal (which I love, but it's a different flavor). The prose is stark and procedural about the mechanics of assassination in a manner the show often glossed over for style.
There's also a deeper dive into the bureaucratic roots of the 'Twelve'. It's less a shadowy cabal and more a depressingly mundane network of ex-KGB and corporate interests, which somehow makes it feel more plausible and grim. Eve's backstory and her marriage to Niko are given more weight early on; you understand her restlessness from the inside out, not just from Sandra Oh's brilliant expressions. Some of the book's plot beats, like the initial Amsterdam encounter, are radically different and much more low-key.
Honestly, the books feel like a bleaker, slower-burning alternate universe. They expand the story by grounding it, paradoxically making the world feel larger because it's less slick and more detailed in its tradecraft and personal damage. The show took the core dynamic and turned up the theatricality and queer tension to eleven, while the novels explore the cost and loneliness of that life with a colder eye. I wouldn't say they're essential for show fans, but they offer a compelling, grittier companion piece if the premise's psychological underpinnings interest you more than the cat-and-mouse glamour.