Now that I have three works available, and a few years of signing events in a variety of venues, I've begun to notice some trends. I'll focus on events since Genre Shotgun has been available.
Overall Sales (rounded off to nearest 5%):
Flank Hawk: 45%
Blood Sword: 15%
Genre Shotgun: 40%
At SF/Fantasy Conventions, Flank Hawk outpaces the other two works.
At venues where it's a repeat appearance for me, Blood Sword's sales rise, most of those sales are people picking up the second novel in the series after obtaining a copy of Flank Hawk the previous year.
When it's a new event for me/first time attending, I get very few Blood Sword sales, even though it's written as a standalone novel in the series, compared both to Flank Hawk and Genre Shotgun. On occasion a reader will pick up a copy of both Flank Hawk and Blood Sword, usually when the reader is very interested in the concept of the First Civilization's Legacy Series.
Even if potential readers seem a little bit more interested in Blood Sword's storyline, readers usually select Flank Hawk, since it's the first novel in the series.
If I am attending an event that attracts a more general audience (festivals, group author events and the like), Genre Shotgun sells better than Flank Hawk, and far better than Blood Sword, of course. At such events, the reader usually picks up Genre Shotgun for his or her reading. When they pick up a copy of Flank Hawk and/or Blood Sword, it's intended as a gift for someone else--son, daughter, grandson or granddaughter (either accompanying them at the event or to be given later).
This has affected my writing of Soul Forge, the third novel in the First Civilization's Legacy Series. It takes a lot of effort to get theplanning and tricky structure just right to make a novel in a series stand alone, complementing the first two for readers of the series up to that point while, at the same time, being novel a reader new to the series can pick up and read first.
Soul Forge will be a standalone work, but not as clearly and cleanly as Blood Sword. Why? I would expect that sales of the third novel will be similar to sales of the second, but to an even greater extent. This sales trend between Flank Hawk and Blood Sword has been consistent since Blood Sword's release.
Online sales, which include the ebook and audiobook versions, and the few brick and mortar sales, are somewhat more murky but seem to largely follow the pattern discussed above. The main difference is that Blood Sword sells much better than Genre Shotgun as an ebook, but not as well as Flank Hawk.
It's difficult to know why because, unless a reader posts a review of more than one of my works, or emails me about my writing, I have no idea who selected the novel and why. It's a situation far different than when I meet customers/readers face to face. Then I have the opportunity to describe my works and discuss their interests and reasons for deciding to obtain a copy (or copies) of my work(s).
This is my observation. Trends for other authors may vary greatly. It'll be interesting to see how Soul Forge and hopefully my science fiction novel, Relic Tech (if/when it gets published) affect sales trends.