One of the wonderful, and terrible-for-budget, things that e-readers have brought us is the moment after you finish a book, and swipe to the next page… only to be greeted with a link to the next in series. Temptation glows, full of electrons and promise of getting the same enjoyment, all over again, for just a couple clicks and a credit card (or with KU, not even that.)

In that moment, in that emotion, we find out yet again that humans are not rational creatures, but rationalizing ones. Each individual is complex and their reasons unknowable to outsiders: the aggregate data, on the other hand, can start to speak to us.

Sell-through (and read-through) comes in two varieties: book-to-book, and series cumulative. At the base, both are very simple equations.

For book to book sell-though, how many people bought book 1 in a given time period (like 30 days), and how many bought book 2? divide the second against the first, and if you have fairly steady sales of both, you have a decent approximation of how many people finished 1 and moved on to 2.

For example, 500 sales Book 2 / 1000 sales Book 1 = 50% sell-through

For book-to-book read-through, first see how many pages your KU book is. This is on your bookshelf page in KDP, under “edit book details”, or you can do a rough approximation by just looking at how many print book pages you have. Then take total KU page reads for that book, divided by the number of pages in that book, to arrive at roughly how many people read it. Repeat for book two, then divide book 2’s quantity by book 1’s quantity, and there’s your percentage.

For example,

100,000 page reads in Book 2 / 200 pages in Book 2 = 500 copies read.
250,000 page reads in Book 1 / 250 pages in Book 1 = 1000 copies read.

500/1000 = 50% read-through

Cumulative sell-through is just looking at the entire series, and going “How many people who started book 1 read all the way through?” So if Book 5 sold 250 copies, then 250/1000 = 25% hung in ’til the end.

The people who buy your first book in series are a very large assortment of folks that happened to get steered your way, whether by word of mouth, weird (or focused) search returns, browsing the hot new book list, advertisements, review sites, also-boughts… a very broad spectrum of humanity. (If you get one-star reviews, this often means you managed to cast a net broad enough the readers included people who aren’t in your target market at all.)

The people who buy your last in series are going to be the core audience that really likes your stuff. So when looking for clues on how to target your marketing, look at the also-boughts last book, not the first, to isolate what your audience really likes.

So, what is a good number?

First, it’s dependent on your marketing strategy. Ideally, it’s over 50% sell-through from Book 1 to 2, and then in the 90’s for the rest of the series. For Kindle Unlimited, it’s 75% read-through (because there’s no monetary hesitation, and fewer clicks so the impulse can be followed more easily.)

However, if you run a deep discount on the price (99 cent promo), then you will see the percentage drop by 10-30%, as a bunch of readers pick up the book, and then take their sweet time getting around to it, or simply move on to the next heavily discounted book of the day.

If you run the first book as free, I’ve seen authors report conversion rates (sell- and read-through) to Book 2 as low as 0.02%, with a high of 6%. It becomes a numbers game: getting enough downloads that the few who go on and read through the series pay for the advertising and turn a profit, on top of that.

So, why are these numbers important?

1.) Advertising effectiveness.

The point of promoting Book 1 in a series isn’t to sell Book 1: it’s to hook the reader for the entire series. After all, if you make a dime on book 1, it’d take a LOT of sales to make advertising cost-effective. But if you make a dime on Book 1 and a dollar each on books 2 through 5, then it doesn’t take near as many sales to make the advertising pay for itself and turn a profit.

2.) Finding out where you lose readers.

If Book 1 to Book 2 has a 20% sell-through when they’re are neither new releases nor subject to large promotions… then you need to look at the cover and blurb on Book 2.

If book 5 to book 6 has a 20% read-through, then it’s time to go back to book 5, and have a friend read the reviews to see what in the book failed to sell the readers on the next book. (And check cover and blurb on Book 6.)

Sometimes you did an abrupt change of emotional tone, or of subgenre, and that’s endemic. It’s not a death knell; Laurell K Hamilton survived a Trigun-level sudden change from urban fantasy to paranormal romance on her most popular series, and continues to sell amazing amounts of books. But it will affect your advertising, and possibly how you package the earlier books.

Sometimes you just needed to fix a few things in edits, add some more foreshadowing in, and make sure that the minor character everyone loved actually got closure on her emotional arc. We’re indies; if it won’t break the book (editing is not rewriting!), then feel free to put that in, so the readers get what they want, and move on to the next one.

Sometimes you can’t fix it, because it’s endemic to the world, the character, and the story. Very well, carry on, and maybe think about starting another series where you can apply the lessons learned the hard way.

(This post brought to you by watching people, after they read my stand-alone latest release, working their way through my back catalog. The numbers wouldn’t be near as easy to decipher if the release had been in a series.)