When you write a series, under ideal conditions you can get some traction with your books and build toward hitting the New York Times Bestseller’s list. In order to hit #1 on the list, you need to do a few things:
1) Generally speaking, you have to have a series that is hot from the start. You can’t have a mediocre book one in the series; the book needs to be strong enough to hook the reader and convince him or her to buy the series, not just the book.
For example, if you look at Robert Jordan’s first novel in the Wheel of Time Series, he has this fantastic prologue that sells the reader not just on the book but on the series as a whole. Though the novel moves fairly slowly in the opening chapters, as a reader you typically will say, “Ah, but I’m in this for the long haul. I want to see how this ends.”
2) The second thing that you need to try to do is to raise the bar in each book. In other words, if book one is great, book two needs to be better. The plot needs to be more brilliant, the pacing better, the writing stronger line-by-line. Readers need to see that you’re not a one-trick pony, but that you can create different effects, arouse powerful emotions, and so on.
3) Normally, publishers want you to put the books out on a regular schedule. If you look at Terry Goodkind’s fantasy series, you’ll see that he consistently put his books out once each year, year after year. Readers trusted him. Several mystery and romance writers have done the same. If a writer stops putting out books in his series for more than a year or two, the readership will often fall off—sometimes precipitously.
Keep in mind that it often takes an author three or four books to hit the New York Times list at all. As the series builds, the author will move up on the extended list—perhaps from the 40’s to the 20’s until they hit in the top ten. The top ten is where you want to be. Sometimes you need to write a long series of books to reach it, but people like Stephenie Meyer and Suzanne Collins prove it can be done in three books or less.