I'm a numbers geek. I like tracking and trending things. Crunching data is one of my self-soothing mechanisms. Twice a year, I get the cold hard feedback on how my books are selling. There are indicators I can follow on a more day-to-day basis: review numbers at Amazon and Goodreads, sales numbers at Amazon (though I can only approximate them based on fluctuations in the sales ranking). But in the final analysis, there's data that's only available from the semi-annual royalty statement.
Sometimes that data is cheering. Like the fact that Daughter of Mystery continues to sell consistently, even 5 years out. It's even still selling consistently in hard copy. (Like, roughly 20% of sales are hard copy.) In the last royalty period, sales of DoM were up from the previous two periods. I credit that to the regular word-of-mouth recommendations the book continues to get in an ever-widening circle. When I look at comparable books released around the same time, I can be pretty proud of the legs my debut novel has.
Sometimes that data is disappointing. It's disapointing to see that not only have sales dropped off with each successive book (which is probably to be expected with any series), but the longer out from release, the worse the sales of The Mystic Marriage and Mother of Souls are compared to the equivalent period for book 1. Both started out by selling about half what Daughter of Mystery did in its first year, but in the most recent period The Mystic Marriage is at a quarter of equivalent DoM sales, and Mother of Souls is at a sixth of equivalent DoM sales. Within a certain fluctuation, Daughter of Mystery is still selling at a fairly consistent rate. The other two are steadily dropping.
Why? It's always hard to guess. For one, I think it's just in comparison with the unexpected legs that Daughter of Mystery has found. When people recommend my books, it's only natural that they recommend the first one. Maybe people pick that one up and don't actually realize there are more? (There are always those who pick up the first book and then don't like it enough to continue, for whatever reason.) Another factor is that succesive books in a series don't get as much attention from reviewers, which can feed into the "there are more?" issue. It's a bit cheering that Mother of Souls saw a big bump in the last royalty period over the previous--quite possibly directly due to being given the Gaylactic Spectrum award--but honestly it would have been hard for it to have done worse than the previous period. So "cheering" is relative.
So, mixed results from this bit of data. But data is good.