Shira Boss has a very interesting piece in this weekend's Times business section on the mystery of making a bestseller.
I don't have any thoughts in particular on what makes a non-fiction bestseller--many of them, it must be said, sound completely vacuous! (I mean, I can see the point of certain guilty-pleasure non-fiction best-sellers, but the self-help genre's pretty much a closed book to me.) But it seems to me that the internet's really changed the dynamic of fiction best-sellers because of the way it so dramatically augments word-of-mouth. No doubt some "bad" novels become best-sellers in any case. But I feel that a small proportion of books really are just much more appealing for one reason or another than most of what's out there. I'm not personally a fan of John Grisham or Dan Brown, but there's a reason those two guys sell so much more than their peers--great sense of narrative pacing is a big part of it. How could Lee Child and Robert Crais not be bestsellers, assuming decent handling at the publishers' end, given the ease and excellence of their story-handling skills? And then, too, there are altogether delightful books--what's a good example?--like, oh, Love Walked In by Marisa de los Santos or Naomi Novik's Temeraire books. I know very little about the original conditions of publication of either of these, how they sold, etc. and yet it was clear to me immediately upon first reading that they had to have fantastic word-of-mouth and a long afterlife. People are going to be reading that de los Santos novel forty years from now from public libraries in the same way they read and reread Georgette Heyer or Mary Stewart. Similarly Naomi's books are going to be read by all sorts of different constituencies--male and female readers of fantasy fiction, fans of Patrick O'Brian, adult female readers more generally who just like her handling of character and voice, young-adult readers--basically as long as people are reading and enjoying novels!
(I am not a pessimist about the future of the novel, I find all the hand-wringing along those lines mildly ridiculous; but I do sort of believe that the world is going to devolve into some Mad Max-like post-apocalyptic scenario in my lifetime. However I feel sure that at that point you'll still be able to pick up a battered paperback or two along with your gallon jug of diesel fuel for the generator and your barrel of water. In any case the paradox of life in the world is that though I do not believe in the future I also funnel all my spare money into a retirement account, so there you go...)
NB irrelevant footnote: When I was little, I was always reading and writing, but from when I was about ten or so I was writing a massive novel called "The Purple Cow" which involved an apartment complex in Vermont built by a man who won a lot of money in the lottery & invited all his extended family to move in with him, they all become preoccupied with writing books and a large part of the story involved tallying up how many books they wrote of what kind in a sort of massive extended-family literary competition (privileging quantity rather than quality)! And I worked on it every day, especially during the summer, and it was always alluded to (very seriously, though not in a real-world sense) by me and everyone else as my bestseller. As in: "Where's Jenny?" "Oh, she's in her room working on her bestseller." It would be very exciting if one day I had an actual bestseller, though I feel that the terminology has become so elastic as to be almost worthless...