Many would-be authors are under the quaint misapprehension that all you have to do to get even the most niche-audience books on bookstore shelves is to publish it.
We know better, don't we. Bookstores have limited space, and that means two important things:
1. They aren't going to keep anything in stock if it isn't selling.
2. They aren't going to even try to stock something they're sure they can't sell. (Yes, it's possible they're wrong about that. Quel dommage.)
"You mean... bookstores refuse to stock a book?" you say. "Isn't that... censorship??"
No, it's business. And bookstores are supposed to know their clientele, and what will make an impression on them and what won't. (Also what will get them so het up they'll stop patronizing the store.)
Sell-in refers to how well the publisher has been able to get books into stores. Good sell-in means retailers have shown enthusiasm for the book and have ordered it in approximately the quantities we had hoped--or better.
Good sell-in is a promising sign of how the book will do in bookstores (because bookstores are pretty good at knowing what their clientele will like), but it's not a sure sign. In a returnable industry, all publishers have had the experience of getting terrific sell-in only to receive most of the books back in returns, because they did not sell through.
Sell-through refers to how well the stores have been able to get books into the hands of consumers. Sell-through (which is difficult to track, except through unreliable Bookscan) means those books have really sold, and are not coming back. We can go to the bank with that money, instead of waiting around with it in our hands, to see if the bookstore is going to demand it back.
It's a tough, tough business, and it's only tougher these days. Do everyone a favor and go buy a book, ok?