Sarah Manguso, 50, is the author of nine books, including two collections of poetry, four works of nonfiction, a volume of short stories and two novels. Her darkly comic memoir The Two Kinds of Decay (2008) charted her experience of a rare autoimmune disease and the years of treatment that brutalised her mental and physical health through her 20s. Ongoingness: The End of a Diary (2015) is an account of her obsessive diary-keeping, and a New York Times Book Review editors’ choice. 300 Arguments (2017) – a collection of aphorisms on desire and failure – was named a best book of the year by more than 20 publications, and her first novel, Very Cold People (2022), was a finalist for the PEN/Jean Stein award. Manguso’s new novel, Liars, documents the end of a marriage. She was born and raised in Massachusetts and now lives in Los Angeles.

Liars, your second novel, was not the book you expected to write…
As I conceived of it, my next book was going to be a period piece set in New England, in the 1940s, and it was going to confront the nuanced and tortured history of who in America got to be “white” and after how many generations. It was roughly inspired by some things I found out about my own family, like the interesting condition of not being born white according to the census, but becoming legally white later in life. That happened to my parents. I’m the first person in my family who was born white. My parents look white, but there are certain ethnicities that, upon arrival, were not considered white.

What was your family heritage?
My mother’s family is Ashkenazi Jew; my father’s father may have been a foundling but was raised Italian and his mother was an Irish immigrant. Teddy Roosevelt said in the first part of the 20th century that, once these southern and eastern Europeans had lived in America for three generations, they were able to be white on the census, and that just seems insane to me – that there were different times at which an Irish person could be white in some states and not in others. There were all sorts of laws in different states and I thought there were so many ways of writing about this in fiction. Then in November of that year, 2020, my former spouse left me with absolutely no warning – he announced he was leaving and was gone the next day. Three or four days after that I started writing, and what ultimately I arrived at was Liars.

You’ve said that Liars is fiction, except for the parts that are true. Did you have concerns about writing something so personal?
It wasn’t that it was personal – I was concerned, above all, that my son not ever have the opportunity to go online when he’s older and be bombarded with a lot of information about his family that he wouldn’t want to confront like that, so I’ve been rigorously careful about what I share about this area of my personal life. My son was eight when his dad left and he knows that his dad cheated on me; beyond that, there’s really not anything more about my personal experience that I wanted to add to that cauldron of facts on the internet. There are some changes I made to the book to appease some very cautious lawyers, but I’m lucky in that I was already a fiction writer when this material fell into my lap. What’s important for me as a writer is that I felt total freedom to write anything, and I don’t think I would have felt even slightly free had I decided to write it as autobiography.

You’ve described the book as “a place to put the rage”. Have you been surprised by the response from women recognising themselves in this story?
I have to confess I’ve been doing something I’ve never done for my other books, which is to go to the Goodreads page and see how the average enthusiastic reader has been responding to this book. It is in equal parts some of the greatest validation I’ve ever received and the most terrifying cultural report I’ve ever read. It’s incredibly disturbing that hundreds of people, in the first week of publication, have said of this domestic abuse novel: “This is exactly what all women have to deal with, thank you.” For the most part, it’s been really interesting to see that I seem to have hit on a cultural sore spot.

Your books have been described as hard to categorise in the way you span genres: poetry, memoir, essays, fiction. How do you define yourself as a writer? Or don’t you?
Early in my career, I would just say that I was a copy editor, which was my day job. I don’t think I was really eager to announce myself as a literary writer; the whole thing seemed embarrassing. I think you’re seen as kind of a weirdo if you publish a couple of books in one genre and then another in a different one; but, once you have 10 books, you’re just getting at things every which way.

What are you working on next?
It’s an erotic novel about ageing, obsession and freedom.

Is the historical novel still on the back burner?
That’s a good question. My editor very delicately said recently: “So is the next one going to be this book that we bought?”, and I had to say, no, but I promise I’ll make something nice for you. I have no idea whether I will ever write that novel. I’m not sure I’m the one to write it. But I never really know what I’m going to write until I’m actually writing it.

What was the first book you fell in love with?
It was an old edition of Grimms’ & Andersen’s Fairy Tales. Half the book was the Grimms’ and then you could flip it and the other half was Andersen. My son has it now, it’s in his room.

Those are dark stories for a child – did that colour your work?
No, I think growing up in New England gave me a pretty healthy acceptance that the gothic is alive and well.

Which author do you think is most unfairly underrated?
I’m tempted to just say “all the women”. I’d say Colette – I think she’s very underrated among anglophone readers, as her name sounds like a little girl’s.

Which modern authors do you read most?
Some writers I particularly love at the moment are Claire Kilroy, Myriam Gurba, Patricia Lockwood and my good friend Sheila Heti.

• Liars by Sarah Manguso is published by Pan Macmillan (£16.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply