Agatha Christie? Really?

I've heard from two sources (both British, so maybe "grain of salt" time) that Agatha Christie is the best-selling author of all time.

Is that true? How can that be?

I have 3 Agatha Christie novels: Poirot Investigates, The Patriotic Murders and Why Didn't They Ask Evans? I must have gotten them on sale somewhere.

These are not what I consider classic literature. Granted, I've heard she wrote 39 books featuring Poirot, so maybe these aren't top quality, but I don't see it.

The format is odd. Everyone but Poirot is written in broad, stereotypical strokes. The story is advanced by sacrificing detail. I'm not impressed. I put Poirot Investigates back on the shelf, and I'm forcing myself to read The Patriotic Murders just to see if it gets any better.

Maybe it's the volume of books she wrote that make her the best-selling. Maybe all British schools require a class on her. Maybe she has a head start on other writers because she wrote in the 1940's.
Maybe it's that the books are short. A 100-page paperback is hardly intimidating. It fits neatly into purse or briefcase for those unexpected waiting room delays.

Whatever. I suppose, to be fair, I should find some of her better-known works, like Murder on the Orient Express, before pronouncing judgment on all her writing. Obviously, if she's The best-selling author, she did something right.

I'm just not sure what it was.