how to write a bestseller

I really need to be packing for Living History Week at Fort Rodd Hill, but so selfless am I that I pause to drink tea and share a revelation with you.
Whenever there's an unexpectedly huge bestseller--The Da Vinci Code, The Girl With the Dragon Tattoo, Twilight, Fifty Shades of Grey--people who enjoy reading or take pride in writing graceful prose, tight plotting and thorough characterisation tend to have their heads meet their desks in a painful and repeated fashion. Why, they ask, why this book, of all the thousands published?

So here's my theory. Which is only about novels, because I don't care about nonfiction bestsellers.

First, the trick of becoming a bestseller is that a book needs to appeal to non-readers. Because the novel-reading public is just not numerous (or uniform) enough to make a breakout bestseller.
 How do you manage to appeal to the non-novel-reading public? By speaking in a voice that is not like a novel, but like something more familiar and comfortable, so they're not startled away.

The Da Vinci Code is famously written in the style of bad journalese, with its 'renowned curator So-and-so', and its flat clunky sentences that cram too much in. It's like reading a second-rate Newsweek article, but that's the key point:  it is not like reading fiction, because reading fiction is scary and uncomfortable.

Twilight and 50 Shades I can consider together, what with one being a fanfic of the other, and thus having a very similar voice (though frankly E.L. James makes Stephenie Meyer look pretty good as a stylist). That voice is not a literary voice at all. It's the voice of your flaky neighbour, or your drama-queen BFF, the one with the complicated love life and massive angst. She's sitting across from you at the kitchen table, drinking coffee and crumbling a muffin into bits because she is too upset to eat. She's telling you about this guy she's involved with, and you are listening in horror and fascination because you would never get into that sort of situation (and if you did you'd handle it better but you can't tell her that). Sometimes you want to shake her and other times you're a little jealous and now and then you think she's got to be kidding. But you keep listening.
And the best part is that she's a book, and so you can shut her up and go have dinner or go to bed. You don't have to keep pouring her coffee until neither of you can sleep, or make up the bed in the spare room because she's too upset to go home or her parents have kicked her out until she stops seeing this guy.  The experience is much better when controllable by the listener.

There's my theory, kids. I'll have to finish Cost of Silver before I can write my blockbuster bestseller, so you can probably get a good head start on me.