Book publishing has long been a hits-driven business. The bestsellers, the logic went, paid for the flops. And it was the authors of those in the middle—the so-called midlist—that publishers hoped to build into the next crop of bestsellers. But midlist sales have faltered enough in recent years that there is a growing concern among publishers and agents about how the business can create new hits when the field they once turned to is, well, disappearing.
Source: Is Publishing Too Top-Heavy?