It All Comes Back to Book Number One
Everyone buys the first book in a series. When you run promotions, they’ll be for the first in the series. When readers ask you where to start, it’ll be with the first in the series. When you calculate the viability of a series based on read-through or buy-through, the first book is your baseline. Everything else will be a subset of that first book’s numbers.
The first book can be longer than the others in the series. That’s better from a revenue perspective because everyone gets the first book. If you participate in the KDP Select (Kindle Unlimited) program, then your page reads from the first book will bring home more revenue for you.
You won’t have more people reading the second book in a series than read the first. Even with a ninety-nine percent read-through rate, you lose one reader out of every one hundred. Anything north of fifty percent is a good number of readers moving to the second book in the series. That means only half of the readers will pick up the second book. (But with future volumes in a series, more people will pick up the first book. You see how the snowball gains mass as it rolls?) From the second to the third, you want a number of ninety percent or higher and from the third to fourth and later books in a series, you want a consistent ninety-plus percent.
And it all comes back to the first book.
Will later books in a series be written better? Probably, but that first book is your exploration of a new idea, the development of new characters, and the world-building that adds color to the backdrop of an exciting story.
First books are written with passion and energy. Readers can tell. The authors can feel it too. Build a better springboard from which to leap higher into your author career.
Let that first book be a beacon for those that follow, a signpost to a great adventure, and most importantly, the moneymaker. Turn it loose, and let it earn its keep (even as a loss leader, which is a different topic, but if it gets readers into the series, you win).
The first book. Everyone always buys the first book.