There’s only so much Come Dine With Me you can watch so, while I’ve been stuck in the house, I’ve been reading a few more books instead. Mainly novels (Fame is the Spur was especially good) and biographies (recommended: Acting My Life by Ian Holm) and some science-fiction, including Station Eleven (be wary though: it’s about a virus that kills you). I’ve also been reading lots of crime fiction.

In particular, I’ve been re-reading some Agatha Christie’s. By The Pricking of My Thumbs for example (why isn’t that novel more famous? the twist at the end is terrifying). And Mrs McGinty’s Dead, another of her lesser-known books that points out that little old ladies aren’t always nice and sweet. And Murder is Easy. Ditto. Every one of Christie’s stories makes the point that we’re all capable of hiding what we think, and who we really are, and often do. I love Christie.

And I’m not alone, obviously. Sales of books in general have increased during the health crisis, but particularly sales of crime, and particularly Christie, and I’ve been wondering whether we should be comforted by this or alarmed. Quite a lot of modern crime fiction is bloody, violent stuff, but Val McDermid once told me that we need the violence of crime fiction. At some level, she said, we all want to be violent and must work to suppress it, and novels are a benign way of doing that.

I’m not so sure though. I’ve had to give up on a few recent crime novels not because of the violence as such but because of violence that lingers, and I think I detect a trend that shows others feel the same. The best-selling whodunnit right now is Richard Osman’s The Thursday Murder Club, which is cosier than a lot of modern crime fiction, and the premise – a group of people getting together to solve murders – reminded me of Christie’s Tuesday Club Murders which is based on a similar idea; 100 years on from the publication of her first novel, her influence is still strong.

But some people still sneer, don’t they? Some people still say the Queen of the Golden Age of crime is cardboard and conventional, but I can’t help feeling they’re missing the point. The Christie novels I’ve re-read remind me how macabre she can be, and dark. Yes, the twists are amazing –The Murder of Roger Ackroyd is surely still the most incredible twist in the history of fiction – but actually it’s often the mood that’s the best thing about the books: the hidden resentments, the hidden weapons, the monsters in dinner suits and ball gowns.

And the idea that Christie is conventional is definitely wrong. I once spoke to James Prichard, her great-grandson, about this and he told me how adventurous she was. “She was a single mother,” he said, “she went to archaeological digs, and some extraordinary places in the Middle East you couldn’t go to now. People call it cosy crime, but I don’t think there was anything particularly cosy about my great-grandmother – she was very forceful.”

I also spoke to Sarah Phelps on the same subject. Sarah has written quite a few of the BBC adaptations and thinks Christie is the exact opposite of cosy: according to her, Christie is subversive and challenging, shocking even. In every book, says Sarah, you have no idea who you’re in a room with: they sound like you, they look like you, and then you pull away their respectability and Englishness and what you get underneath is somebody who is completely different and terrifying. Some may think Sarah’s interpretation goes too far and many will continue to dismiss Christie as a serious writer, but I think Sarah is on to something and it may help explain why Christie is so popular. It may also explain why she’s being read so much during a crisis caused by a virus that is, to use Sarah’s words, different and terrifying. Christie’s sales (two million in the English-speaking world last year) also pretty much put her beyond the reach of the doubters anyway.

But there’s something else as well. Many modern crime novels put procedure, and science, and DNA, at the heart of their books and that’s fine, but Christie is different. The route to a solution in her books is always through pure thought and in particular contemplation of the nature of human beings. Right at the end of his very last story, Poirot, frail and in a wheelchair, points to his head and says: “The brain… magnificent as ever” and that’s the point. It was never really about the knife, or the gun, or the blood in Agatha Christie’s books. It was always about the brain.